CI: Mark s3 tests parallel safe
Closes https://github.com/pandas-dev/pandas/issues/35856

I think we need to update the pytest pattern though, so this should
fail.
> so this should fail.

Does this mean that this PR should not be merged despite being green? I'm going to rerun it a few times. I suspect we'll get a failure. Updated to just make these fixtures parallel safe. At the start of each session, each worker starts up a moto server on `localhost:555{worker_id}`. Then we just need to plumb that through everywhere.

I think Martin mentioned possible memory issues on windows workers with this approach, but that may have been starting the moto server at the test level. Hopefully doing things at the session level avoids that. Rerunning azure at https://dev.azure.com/pandas-dev/pandas/_build/results?buildId=40904&view=results. But we should be good. https://github.com/pandas-dev/pandas/pull/35655 wasn't backported, so this shouldn't need a backport either. I've seen a couple failures on re-runs, but nothing to do with s3 / moto. So this should be good to go. > I think Martin mentioned possible memory issues on windows workers

Yes, this was for the case of restarting the process for every test (i.e., standard pytest scope) - I believe moto may be less than optimal at cleaning up on exit, at least on Windows.