Resolve pytest warnings about the deprecated --strict flag

[Zalathar]

(New version of #2862; fixes #2863.)

[Zalathar]

I'm not sure if the coverage invocations in tox.ini actually need an explicit --strict/--strict-markers, but I didn't want to spend the time to investigate so I just converted them. 🤷‍♂️

[Zalathar]

If #2865 lands then I'll be able to get rid of all the workarounds for pytest 4.3.

[Zac-HD]

Thanks @Zalathar 🎉

[sobolevn]

I am a bit late to the party, but --strict-config also looks nice: https://docs.pytest.org/en/stable/deprecations.html#the-strict-command-line-option

Should we add it as well?

[Zac-HD]

Tempting, but it's not supported on 4.6! IMO we can run that occasionally by hand, and go back to --strict once that's re-supported as an all-in-one flag.

[Zalathar]

The key limitation is that we can’t add relatively-new flags to pytest.ini, unless we commit to a slightly annoying override in the legacy-pytest task.

(The override is honestly not that bad, so I’m fine with it if a new flag adds significant value, but I’m not convinced that --strict-config clears that bar.)