Allow type narrowing with is_async_test

[seifertm]

This PR adds additional information for the type checker to is_async_test(Item). This allows the type checker to narrow the type and avoids subsequent assert statements or the use of typing.cast.

In order to use typing.TypeIs, the dependency on typing-extensions is extended to more recent Python versions.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 90.86%. Comparing base (7ca56e3) to head (439dfaf).

Files with missing lines	Patch %	Lines
pytest_asyncio/plugin.py	80.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1213      +/-   ##
==========================================
+ Coverage   90.81%   90.86%   +0.04%     
==========================================
  Files           2        2              
  Lines         403      405       +2     
  Branches       43       44       +1     
==========================================
+ Hits          366      368       +2     
  Misses         29       29              
  Partials        8        8              

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