Complete static typing

[TeamSpen210]

This adds type hints to all definitions in the package, as well as the tests. It passes mypy's strict mode, with some caveats:

• _exceptions will break if you use newer trio versions due to the removal of MultiError.
• compose_values() is untypable due to it creating new named tuples with every call. Therefore the attribute names aren't checked, but they produce the right value, and it is a tuple subclass.
• Like the sync versions, azip()/azip_longest() just accurately types up to 5 arguments.
• Mypy version is tricky. 1.4.0 is the latest supporting 3.7, but it gives some spurious errors. 1.8.0 is the last to drop support for checking 3.7 code, which does work, but not with the current CI setup. Not sure what to do here...

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (c908bc7) to head (bb1b0ba).

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Additional details and impacted files

@@            Coverage Diff            @@
##            master       #24   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           15        15           
  Lines          454       528   +74     
=========================================
+ Hits           454       528   +74     

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