Restore python 3.6 support

[pavera]

This PR restores python 3.6 support by allowing version ranges for pip and pip-tools. The latest versions of these dependencies no longer support python 3.6, so we need to allow earlier versions.

Looking into an issue creating PRs for a python repo found that if you specify a python version, IE 3.6.8 for your repo, then when we attempt to create the 3.6.8 pyenv and pip install -r python/helpers/requirements.txt it fails because some dependencies in this helper have removed python 3.6 support in their latest versions.

I'm not clear on whether dependabot will just immediately try to bump these versions again? If it will what is the correct way to pin these versions back?

I'd like to add tests as well, but need a pointer or 2 about where to add such "integration" type tests.

[jakecoffman]

This makes sense now that I learned the Python helpers are run with the target version of Python.

Many of the spec are integration style tests already, they make calls to GitHub and compile external code! So maybe stick a new test in pip_compile_file_updater_spec.rb?

Also, I noticed this file

dependabot-core/python/lib/dependabot/python/python_versions.rb

Lines 12 to 19 in 48f4df6

	SUPPORTED_VERSIONS = %w(
	3.10.3 3.10.2 3.10.1 3.10.0
	3.9.10 3.9.9 3.9.8 3.9.7 3.9.6 3.9.5 3.9.4 3.9.2 3.9.1 3.9.0
	3.8.12 3.8.11 3.8.10 3.8.9 3.8.8 3.8.7 3.8.6 3.8.5 3.8.4 3.8.3 3.8.2 3.8.1 3.8.0
	3.7.12 3.7.11 3.7.10 3.7.9 3.7.8 3.7.7 3.7.6 3.7.5 3.7.4 3.7.3 3.7.2 3.7.1 3.7.0
	3.6.15 3.6.14 3.6.13 3.6.12 3.6.11 3.6.10 3.6.9 3.6.8 3.6.7 3.6.6 3.6.5 3.6.4 3.6.3
	3.6.2 3.6.1 3.6.0 3.5.10 3.5.8 3.5.7 3.5.6 3.5.5 3.5.4 3.5.3
	).freeze

Should we test back to 3.5.3? 🤔

[pavera]

Yeah, I also had read the version file, going back to 3.5 pulls these two packages back quite a ways. I figured I'd wait and see on that. 3.6 is already eol, but ubuntu 18.04 is still supported and that is the official python version there. Not sure if there are any supported distros that are keeping 3.5 alive still (rhel/centos maybe?)

Good to know about the tests, I will endeavor to add one for this case.

[jakecoffman]

Looks good, nice work! 👏