Resolve pytest warnings about the deprecated --strict flag

[Zalathar]

As of pytest 6.2, --strict is a deprecated alias for --strict-markers, which was renamed to reflect the flag's actual narrow behaviour.

https://docs.pytest.org/en/stable/deprecations.html#the-strict-command-line-option

This gets rid of an annoying warning that would appear when running pytest, making it harder to view the actual test results.

[Zalathar]

(I basically just did a search for --strict[^-] in all files, so hopefully this caught them all.)

[Zalathar]

See also:

• Make --strict imply --strict-config / xfail_strict? pytest-dev/pytest#7503
• Deprecate --strict  pytest-dev/pytest#7530

I do think we should try to enable a strict-everything flag when it exists, but for now there's no benefit in having warnings all over the place.

[Zalathar]

Oh, it looks like this (understandably) fails in check-pytest43. I'll have to see if I can do something about that.

[Zalathar]

Closing this for now in favour of #2863.