[WIP] Add a Python type checker using MyPy

[cpitclaudel]

I think we'll hold this until we merge the next-checker stuff :) This is a checker for MyPy. Types!

[dakra]

Any news on this? I would like mypy support :)

[cpitclaudel]

@dakra Can you try the branch and report whether it works for you? That would help :)

[dakra]

I tried your changes and seems to work fine.

I had to rename a few command line arguments:
--suppress-error-context is now --hide-error-context
and instead of --silent-imports I always use --follow-imports=skip (since we never want to return errors from different files?!) and and replaced the original --silent-imports with --ignore-missing-imports.

I didn't play around with it too much but lgtm 👍

[fmdkdd]

@cpitclaudel Do you still want to wait for the next-checker changes? I'd rather have that working now than in an hypothetical future ;)

[fmdkdd]

Closing in favor of #1354.