bpo-44451: Reset DeprecationWarning filters in test_importlib.test_entry_points_by_in...

bpo-44451: Reset DeprecationWarning filters in test_importlib.test_entry_points_by_index (GH-26784)

This avoids the following error if DeprecationWarnings are ignored.

    ======================================================================
    ERROR: test_entry_points_by_index (test.test_importlib.test_metadata_api.APITests)
    Prior versions of Distribution.entry_points would return a
    ----------------------------------------------------------------------
    Traceback (most recent call last):
      File "/builddir/build/BUILD/Python-3.10.0b3/Lib/test/test_importlib/test_metadata_api.py", line 145, in test_entry_points_by_index
        expected = next(iter(caught))
    StopIteration
    ----------------------------------------------------------------------
    Ran 1402 tests in 2.125s
    FAILED (errors=1, skipped=18, expected failures=1)