reenable the warnings plugin

[graingert]

The warnings plugin was disabled in #4056 but that issue is now fixed by
pytest-dev/pytest#2445

[nateprewitt]

Thanks Thomas, I'll kick the failed test once the rest finish. If they're all working this seems fine.

[nateprewitt]

Hmm, the failure seems reproducible but unrelated. It's localized to Pypy with:

Fatal Python error: Aborted

Stack (most recent call first, approximate line numbers):
  File "<frozen importlib._bootstrap>", line 220 in _call_with_frames_removed

We haven't made any changes that would have affected this, so I'm assuming it's either a GH CI issue or an underlying test depenency isn't playing nicely with Pypy anymore.

I'm fine ignoring for this PR while we track that separately.

[graingert]

Looks like the 18.04 runners aren't getting picked up anymore: #6417

[graingert]

Hmm, the failure seems reproducible but unrelated. It's localized to Pypy with:

Fatal Python error: Aborted

Stack (most recent call first, approximate line numbers):
  File "<frozen importlib._bootstrap>", line 220 in _call_with_frames_removed

We haven't made any changes that would have affected this, so I'm assuming it's either a GH CI issue or an underlying test depenency isn't playing nicely with Pypy anymore.

I'm fine ignoring for this PR while we track that separately.

@nateprewitt I think failing due to trustme's dep on cryptography #6419 (comment)

[graingert]

@nateprewitt yep it was cryptography, here's the fix #6421

[nateprewitt]

Thanks again Thomas for taking the time to debug the chain of issues to get the PR working. Really appreciate it! I'll merge after tests finish and we should be good now.