Add reproducibility test for `gzip.compress()`

The issue https://github.com/python/cpython/issues/112346 was reported, which was a failure to keep reproducibility in gzip module output from Python 3.10 to 3.11.

There was a fix discussed and rejected in https://github.com/python/cpython/issues/112346

I think it makes sense not to actually change things back, but failing to merge the PR means we also failed to add unit tests.

Can we please add a general-purpose, fragile reproducibility test for the gzip module, so that this problem does not recur?

Ex. test that the output of gzip.compress("Hello world") remains stable

def test_gzip_reproducibility():
    """If this test fails, gzip.compress does not give reproducible output. Either fix that, or document the change in What's New and change the constant 'expected' below."""
    expected = b'\x1f\x8b\x08\x00\xa2\xd2\xe1g\x02\xff\xf3H\xcd\xc9\xc9W(\xcf/\xcaI\x01\x00R\x9e\xd6\x8b\x0b\x00\x00\x00'
    assert gzip.compress(b"Hello world") == expected

    expected = 
b'\x1f\x8b\x08\x00\x00\x00\x00\x00\x02\xff\xf3H\xcd\xc9\xc9W(\xcf/\xcaI\x01\x00R\x9e\xd6\x8b\x0b\x00\x00\x00'
    assert gzip.compress(b"Hello world", mtime=0) == expected    

Linked PRs

• gh-151214

Review linked PR gh-151214 and locate the existing gzip module tests before starting. Add the reproducibility checks for gzip.compress() shown in the issue, then confirm the expected outputs remain stable and account for any intentional change in What's New.