Add Mypy support

[ascandella]

Mypy (https://mypy.readthedocs.io/en/stable/) is a static type checker for Python. It lets you optionally type your code using type hints, which are basically ignored by the Python runtime.

Note that mypy runs a little differently than some of the other linters -- because it needs to understand your imports, you generally run it on an entire project, not just a set of files. The way I wrote it is that it ignores the extensions configuration option, and defaults to running mypy against the project directory. If you want to run mypy against a subset of your project, you can pass the directory name(s) as mypy_args and then the linter will skip adding the project directory.

I got some baffling windows failures on test runs, leading to me changing the way python requirements are installed during the github test action run. I still don't understand why this fixes it, but you can see in the last two commits of ascandella#1 that installing as one shot fixes it:

Broken: https://github.com/ascandella/lint-action/runs/434789642#step:9:57
Working: https://github.com/ascandella/lint-action/runs/434838676#step:9:1

I don't have a windows machine available to me to debug further.

[ascandella]

Here is an example on a test repo I created: https://github.com/ascandella/mypy-action-test/pull/3/files#diff-5bc02cefb3ea9e27f1a6776eabd1935dR5

[samuelmeuli]

Really nice work, thanks a lot for your contribution! 🙂

[ascandella]

Awesome, thanks for your project and the quick responses, it's very useful!