Convert ResourceWarnings into errors during tests

[dstansby]

python by default ignores these, but we probably want to catch them in our tests (and there's a couple I spotted looking through the logs recently).

The full list is

ignore::DeprecationWarning
ignore::PendingDeprecationWarning
ignore::ImportWarning
ignore::ResourceWarning

Do we want to error on any of these other ones?

[nabobalis]

Fixing these would be very good.

[dstansby]

🤔 this did not trigger test errors like I thought it would

[dstansby]

The lack of errors might be due to pytest-dev/pytest#5676

[dstansby]

Looks like when pytest-dev/pytest#8055 winds its way into a pytest release we can revisit this.

[dstansby]

I don't think we need this any more, as ResourceWarnings seem to be caught automatically by pytest 6.2

[kakirastern]

🎉