[WIP] First pass of mypy-cleanliness

[oremanj]

The goal of this PR is to make mypy -p trio work.

WIP because there are still some errors:

• "cmp is deprecated, use eq and order" thrown by attrs plugin: will be resolved by upgrading attrs
• Path needs a stubs file
• 3-4 others that I ran out of time to fix on the flight

And we should add mypy to CI once it's working so it stays working.

[pquentin]

"cmp is deprecated, use eq and order" thrown by attrs plugin: will be resolved by upgrading attrs

We need a release of python-trio/outcome#25 before we can fix attrs in trio proper

[pquentin]

Okay, the attrs fix has landed in master. :)

[oremanj]

Superseded by recent work