Pytest 8.4 `pytest.PytestReturnNotNoneWarning` is now an exception

[rossbar]

The recent release of pytest 8.4 has begun to cause failures in the pytest_mpl tests in the networkx test suite (networkx/networkx#8090).

AIUI, the crux of the problem is that pytest now raises whenever a test returns a non-None value, which conflicts with the design of the mpl_image_compare mark. See also pytest-dev/pytest#13477 for further context/links/discussion.

[ConorMacBride]

Thanks @rossbar for letting us know that this warning is now an exception. There are some more details in this section of the docs, but basically when pytest-mpl is installed it'll always intercept any returned figures and prevent this exception from being raised. Alternatively the tests could be skipped. (See also #183)

[ConorMacBride]

Closing as this behavior has now been reverted in Pytest 8.4.1 pytest-dev/pytest#13495