MAINT: Python 3.10 and Windows -- a few tasks

[tylerjereddy]

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Based on some temporary Windows-only workarounds in gh-3461 we eventually need:

• [ ] to remove the warning filters placed on test_easy_trigger_warning and test_flag_index_error, which are apparently related to the use of six somewhere in our dependency chain (I would expect a library of compat shims for Python 2 to be something that we/the ecosystem can shed sooner than later...)
• [ ] to remove the xfail placed on test_write_with_drivers for h5py; I note that this xfail has cycled between usage and being commented out in the past, so the test/h5py clearly still needs to refine this

[IAlibay]

Just to add here - the six issues aren't limited to Windows, they also affect linux, see #3458

note: it's test_warning instead of test_flag_index_error that is problematic here, for that one as per #3458 my suggestion is just to drop the check on the number of warnings (it's not necessary in my opinion)

[hmacdope]

@IAlibay @tylerjereddy did we end up closing these out?

[tylerjereddy]

Looking at the source of latest develop, test_easy_trigger_warning still has some weird warning stuff going on, so we should probably check that. test_write_with_drivers still has the Windows xfail, which we should also likely revisit.

Neither seem critical though.