Python: Add tox commands for code format and type checking #3282

[cabhishek]

• This PR adds tox commands to automate code formatting and type checking.
• tox uses following

• black for code formatting
• isort for import ordering
• autoflake for removing unused imports
• mypy for static type checking

• Core tox commands

• tox will run tests and linters (code format and type checks). This is the central command that'll be most used.
• tox -e linters will check if the code adheres to the standard and passes type checking
• tox -e format automates code reformatting

• Future enhancements

• Add tox commands for documentation creation
• Perhaps add git pre-recommit hooks to make it more seamless?

• We'll need a follow PR to run these commands over the codebase.

[cabhishek]

One open question: Should we pin black and isort to a version to ensure we don't constantly introduce newer formatting rule changes that are unrelated to PRs? We could periodically upgrade versions and open PR specific to formatting changes. This will ensure we don't mix logic change vs pure formatting.

[samredai]

@cabhishek I think pinning makes sense!

[jun-he]

Yep, I think we should have a requirements file for pinning.

[jun-he]

QQ, wondering if we should support 3.6? Per recent sync meeting, the plan is to support just py37,py38,py39 or higher versions.

[samredai]

I would vote that we don't support 3.6 unless someone really feels we should. Looking at endoflife.date/python shows that python 3.6 security support ends in a little over a month.

[rdblue]

I think that we discussed this in one of the python syncs and we agreed that supporting 3.6 was a bad idea.

[cabhishek]

Reverted changes and dropped 3.6 support.

[samredai]

One small nit comment that I don't think needs to block merging this PR. LGTM! Thanks @cabhishek

[samredai]

nit: Should we add --no-implicit-optional here? This will require the explicit x: Optional[int] = None and will fail on x: int = None. The latter always reads odd to me, "must be an int, but by default it's None". The Optional type hint feels clearer.

[cabhishek]

Added the flag. And given how common it is to have code like x: int = None I guess the future version of mypy will make that flag the default.

[rdblue]

Thanks, @cabhishek!