Fix typing and update to mypy 1.8.0

[bluetech]

When reading the code I noticed the line _event_loop_fixture_id = StashKey[str] which should be _event_loop_fixture_id = StashKey[str]() (an instance). I wondered how come mypy didn't complain, and the reason is that pytest wasn't installed in the pre-commit mypy environment.

This PR:

• Adds the missing pytest dep
• Fixes the resulting typing errors
• Upgrades mypy to the latest version
• Fixes mypy tests/ by makes tests/ a package (adding __init__.py files)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (e92efad) 94.33% compared to head (a53f378) 94.35%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #769      +/-   ##
==========================================
+ Coverage   94.33%   94.35%   +0.01%     
==========================================
  Files           2        2              
  Lines         477      478       +1     
  Branches       95       96       +1     
==========================================
+ Hits          450      451       +1     
  Misses         17       17              
  Partials       10       10              

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[seifertm]

This is great! Thank you for the patch.

I'll rebase and add a changelog entry, so we can get this out in a pre-release right away.