Deprecate @pytest_asyncio.fixture for synchronous fixtures

[SaInekK]

Using @pytest_asyncio.fixture on a synchronous function is a user mistake — synchronous fixtures should use @pytest.fixture. Previously this silently succeeded, making the error hard to diagnose.

This PR:

• Widens _R = TypeVar("_R") (removes the Awaitable | AsyncIterator bound) so applying @pytest_asyncio.fixture to a sync function no longer raises a type error before the warning fires.
• Emits PytestDeprecationWarning at decoration time when the decorated function is not a coroutine or async generator.
• Adds tests for both the bare-decorator and factory-decorator paths.

Closes #1090

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.54%. Comparing base (7817bdf) to head (03c9ab8).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1473      +/-   ##
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+ Coverage   94.50%   94.54%   +0.03%     
==========================================
  Files           2        2              
  Lines         510      513       +3     
  Branches       62       63       +1     
==========================================
+ Hits          482      485       +3     
  Misses         22       22              
  Partials        6        6              

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[seifertm]

The PR relaxes type constraints on pytest_asyncio.fixture in favor of raising a PytestDeprecationWarning. Consequently, synchronous fixtures decorated with pytest_asyncio.fixture no longer result in a warning from the type checker, but instead emit a warning at runtime.

While the submitted patch technically solves the linked issue, the chosen approach defeats the purpose of typed code, which tries to catch issues in a static analysis step as opposed to at runtime. Users will also lose typing errors for the deprecated use in their IDEs and editors. Unless you can convince me otherwise, I'll close this PR.