Fix black formatting failure on tokenize.pyi

[stroxler]

Summary

I noticed this file was mal-formatted when using black . to
autoformat a different branch.

This was confusing to me, I rely on autoformatting after writing
messy code. The reason for it appears to be that black and isort
do not agree

To be specific:

• even with profile = "black" set, it seems that isort will add only
  one blank line on .py files unless otherwise specified. This is in
  direct contradiction to what's stated in isort github issues and docs,
  which claim that profile = "black" should solve the problem on newer
  versions of isort (I tried against the latest and it makes no
  difference)
• But if we use force blank_lines_after_imports = 2 as the
  pyproject.toml currently does (and is necessary to not mess up the
  majority of our code in *.py files, then isort and black will
  argue with one another about how many blank lines to use in .pyi files

My solution is to skip isort on *.pyi files, which I think is better
than having confusing behavior.

As an alternative, or in parallel, it might be worth me opening a PR
demonstrating that profile = "black" isn't working for us and
referencing it in an issue in isort.

Test Plan

python -m unittest
black .
isort .

[stroxler]

Closing - this PR is correct in that black doesn't like our stubs file, but if I autoformat it then isort rejects the change.

Seems like a configuration issue, I can look into whether there's a way to make them agree

[stroxler]

Reopening with a fix that disables isort on .pyi files

As discussed in the summary we could also try to raise this issue upstream - the isort team seems to think they closed this bug but their fix isn't working for me

[stroxler]

On second thought, leaving closed, I think #515 is a better approach