Fix all warnings when running tests

[Lotram]

When we run the test suite, we have a lot of noisy warnings. Most of them are DeprecationWarning of third party packages, so we could safely ignore them, others are valid warning that could be fixed. To avoid having such warnings (and do nothing about them), we could also ensure tests are failing when there are some warnings (pytest has an option for that).

Pytest doc about warnings

On a related subject (but that's open to discussion), I think tests could benefit from migrating completely to pytest.

[ryneeverett]

Most of them are DeprecationWarning of third party packages, so we could safely ignore them

Practically speaking, that means they're coming from dependencies which are unmaintained, which is something we can't safely ignore.

It looks to me like every single warning is either valid and can be fixed within bugwarrior or would be fixed by #1078 and #1134.

On a related subject (but that's open to discussion), I think tests could benefit from migrating completely to pytest.

Please open a separate issue elaborating the benefit.