Update and pin mypy and add CI

[rchl]

Adding Sublime's Text own Python Lib directory broke mypy with an error like:

/Applications/Sublime Text.app/Contents/MacOS/Lib/python33 is in the MYPYPATH. Please remove it.
See https://mypy.readthedocs.io/en/stable/running_mypy.html#how-mypy-handles-imports for more info

Instead of including ST's Lib directory, bundle ST stubs.

[rchl]

To be fair I don't really understand why old approach broke mypy.
In theory these days it would be better to use ST's own files as that would be always up to date and those include types now...

The link doesn't really explain this issue very well and even quick glance at mypy's code is not making it clear.

@rwols @jfcherng @predragnikolic any idea?

[rchl]

Looked at mypy's code a bit more and I think I understand it now.

Mypy doesn't like when path in MYPYPATH is already included in PYTHONPATH. I need the path in PYTHONPATH so that Jedi can analyze types so the solution in theory is to only add paths to PYTHONPATH. And that sort of works, Mypy can see the types too but then it complains about another type-related thing:

And https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-library-stubs-or-py-typed-marker explains why it behaves like that...

[rchl]

The initial error is still there with those changes because the paths are still duplicated...

This is so stupid, mypy shouldn't be complaining about that...

[rchl]

Seems like the whole issue is just a regression in mypy...
python/mypy#7604 (comment)

[predragnikolic]

Will we close this PR because this is a regression in mypy?

[rchl]

There are some useful bits here still and also I'm not sure if I should pin mypy version to resolve this issue for existing users...

[rchl]

But I guess I will not go with bundling external types if I don't have to.