Add Tests, CI, and Matrix for different Django and Python versions, Fix compat issues found in Matrix

[agilgur5]

PR Initial Summary

Need to write-up a real PR description for this elaborating on all my struggles with Poetry and versioning bugs found everywhere. Will edit this later, gonna go to bed soon. This ended up taking a lot longer and being a lot bigger than I expected/intended for 😅

One thing I wanted to note was that I need to add a CONTRIBUTING.md to this PR now that there's a bunch of stuff involved in Development and some confusing things (e.g. Poetry not doing the Packaging/Publishing, just used for dev).

But yea 92% test coverage across multiple versions of Django. And some bugs found and fixed. Will release v0.0.6 with this, but this library is now ready for Beta at least.

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EDIT: Here's the real PR description, almost a month later (it's been extremely busy 😕 )

PR Description

This PR was originally just supposed to add like a single test for latest Django. It turned out a lot bigger than that.

DevDeps set-up

For one thing, adding testing means adding dev dependencies -- there are currently no actual dependencies (Django is a peerDep), so this means adding dependency management as well. requirements.txt used to be the norm, but I was looking at installing via tox as well since we will (and now do) have multiple versions to test against. I forget how I ended up at pipenv, but that's what I started using for dependency management, and immediately I ran into issues during basic installation. Due to pypa/pipenv#2396, installing a new dependency doesn't quite work the same as it would with, say npm. This was already causing me some hassle and can be done automatically, so I started looking for an alternative solution and remembered reading about poetry some time ago.

And yay, Poetry has some much more intuitive and automatic usage, particularly for semver users. Poetry is not without its own bugs though, as I immediately hit this issue: python-poetry/poetry#534 (comment). But once I fixed that, it worked all great until we got to multiple versions.
I was not able to use it to replace the publishing system of setup.py, MANIFEST.in, setup.cfg quite yet though. Some of the issues I ran into on this front (see first commit) might be fixed in the 1.0.0 prereleases however (may be worth upgrading).

Once I started adding testing dependencies I ran into some nuances with Poetry. If you don't specify multiple constraints, you will get errors when installing as one version of a dep generally won't fulfill both Python requirements -- so had to pin to ^3.5. Pinning a Python version also changes the publishing behavior, even if that version is only necessary for dev deps.

Test harness set-up

Got through adding the test harness and first tests rather painlessly, though it took a bit to figure out the proper testing structure/directories and the minimum necessary Django files for the tests to actually run. Very helpful that poetry install also does an editable install of the package being built as well. Had to learn some of the nuances of how the testing and coverage defaults in Pytest differ from, say, Jest, and had a bit of confusion when adding testing settings (e.g. thought I could put everything in .coverragerc instead of pytest.ini, but that didn't work). Adding simple CI & CodeCov went fairly smoothly.

Multi-version tests

And then we get into the real time-consuming and big issues when I decided "hey might as well throw in extra tests for different versions in this PR as well!" and there went all my time into a debugging void 😅

For one thing, multiple constraints totally did not work in Poetry, at least not for devDeps: python-poetry/poetry#666 (comment) . So from there, I had to either do manual poetry add -D in CI commands, which only worked while all other constraints were satisfied by said deps, or just use straight pip install with pinned version numbers of all packages 😕 😕 . That's pretty far from ideal and is similar to using Tox. I was hoping I could use Poetry to manage dependencies across multiple versions of Python / Django, etc with all its benefits and tracking, but I couldn't, so had to do it a bunch of version and compatibility tracking manually. That meant I couldn't use poetry for finding correct dependency versions, installation, or testing, which is quite a few lost benefits. It also means that I can't remove setup.py once Poetry is used for publishing because it's needed for editable install when using pip directly in these different environments.

I also didn't use Tox because of reading various difficulties from a few places of integrating it with Travis. The downside is that a bunch of tests can only run in CI now, so I might want to change this in the future.

Poetry also doesn't quite support Python 3.4 (python-poetry/poetry#1223) despite saying that it does, and Django 1.5 can't use Python 3.5 (https://stackoverflow.com/questions/34827566/attributeerror-module-html-parser-has-no-attribute-htmlparseerror/36000103#36000103)

Also, a multiple constraint on something other than the Python version, for instance, on the version of Django, just caused Poetry to infinite loop and eventually error out.

Django 1.5 testing

The Django 1.5 test was incredibly difficult to figure out all the compat issues for.
Like we need to drop down to pytest-djangov2.5 for it to work with Django 1.5.
And then I got some really cryptic errors that were actually compatibility issues with pytestv3.6+, so I had to downgrade all the way down to pytestv3.5. The RemovedInPytest4Warning of course don't occur anymore in Pytest 4+, and especially not in Pytest 5+, hence some cryptic warnings. pytest-django also doesn't directly depend on pytest, so there's no error during installation from pip either.
pytest-cov then failed to install. This was at least a clear error, but it was hard to figure out which version would work until I found this commit and downgraded to pytest-covv2.6. The changelog for 2.6.1 doesn't quite mention that support for pytestv3.5 was dropped

Summary here is that lacking semver and peerDeps support in Python-land are missing quite a bit and that can cause quite a bit of issues

Python 2.7 testing

Fortunately this only required pytest<5 and this was written in its changelog.

As pointed out in the commit adding this test, I did get an incredibly weird, reproducible issue where editable install read from pyproject.toml despite this being Python 2.7 and pip using setup.py instead anyway. Reproduced here: https://travis-ci.org/agilgur5/django-serializable-model/jobs/584822609

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@b0cbb8d). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master       #6   +/-   ##
=========================================
  Coverage          ?   91.78%           
=========================================
  Files             ?        1           
  Lines             ?       73           
  Branches          ?       15           
=========================================
  Hits              ?       67           
  Misses            ?        4           
  Partials          ?        2

Impacted Files	Coverage Δ
django_serializable_model.py	91.78% <100%> (ø)

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[agilgur5]

A few changes needed already. Also, as said above, need a CONTRIBUTING.md

[agilgur5]

Had one final look-through, and think it's all good to go now... finally!

Decided to add a maidfile.md right now instead of a full CONTRIBUTING.md as that'll take some time to write well and intentionally to give potential contributors and potential novices the right impression. I also don't have a CONTRIBUTING.md (or issue or PR templates, or a Code of Conduct for that matter) in any of my repos yet, so it'll be a first for me and can't just copy it from another of my repos (but can use other folks' for inspiration)

Also added a small README to test_project to clarify some things.

One last thing missing is just a publish commit!