Allow AsyncIterable as a return type for async generator fixutures

[seifertm]

This PR adds AsyncIterable to the supported return values of async generator fixtures decorated with pytest_asyncio.fixture.

The patch also simplifies the type signature of pytest_asyncio.fixture by removing superfluous Any specifiers for generic types, extends type checking to test code, and wraps remaining Hypothesis tests into pytest.Pytester to avoid requiring hypothesis as a type checking dependency.

Closes #1090

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.50%. Comparing base (bf5b4e0) to head (b3cd305).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1481   +/-   ##
=======================================
  Coverage   94.50%   94.50%           
=======================================
  Files           2        2           
  Lines         510      510           
  Branches       62       62           
=======================================
  Hits          482      482           
  Misses         22       22           
  Partials        6        6           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.