Updated checks that no warnings were raised

[radarhere]

A warning has started appearing.

https://github.com/python-pillow/Pillow/runs/5266944375#step:8:3064

PytestRemovedIn8Warning: Passing None has been deprecated.
See https://docs.pytest.org/en/latest/how-to/capture-warnings.html#additional-use-cases-of-warnings-in-tests for alternatives in common use cases.

It is complaining about our method for checking that no warnings were raised during a test.

with pytest.warns(None) as record:
    ...
assert not record

Instead, https://docs.pytest.org/en/latest/how-to/capture-warnings.html#additional-use-cases-of-warnings-in-tests suggests

with warnings.catch_warnings():
    warnings.simplefilter("error")
    ...

I have implemented that here, albeit without the simplefilter, since I presume we would also like to know if any errors are raised.