Apply AnyIO upgrade changes only on test suite

[Kludex]

• Closes No module named 'exceptiongroup' when upgraded to starlette 0.31.0 #2227
• Closes tests/middleware/test_wsgi.py::test_wsgi_exception fails with anyio 3.7.1 #2228

[Kludex]

This isn't supposed to raise an error. I'll investigate later.

[agronholm]

Are you sure this is the right solution? You told me yourself that you'd prefer the singular exception from the exception group to be extracted and reraised. If you don't want to add exceptiongroup as a dependency yet, I think the exceptiongroup import just needs to be wrapped with a try...except ImportError. Then we can replace the context manager with a no-op.