MAINT: bumping minimum required pytest version and fixes coverage job

[bsipocz]

This should fix the CI failure for the oldestdeps job (a newer version was picked up there, and there is really no point in forcing the plugins to have older versions for the sake of keeping the 6.0 in place.

I couldn't reproduce the devdeps failure locally, but will iterate on it in this PR.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.87%. Comparing base (fd08e21) to head (d7cdfd7).
Report is 10 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3337   +/-   ##
=======================================
  Coverage   69.87%   69.87%           
=======================================
  Files         232      232           
  Lines       19770    19770           
=======================================
  Hits        13815    13815           
  Misses       5955     5955           

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[bsipocz]

A warning from pytest-cov now raised as an exception, while it was silently ignored when using pytest<8.4 (pytest-dev/pytest-cov#693 (comment)).

It's not clear which other plugin triggers it, I suspect it maybe related to getting the version info for pytest-astropy-header; but either case, it doesn't matter, we really just can ignore it.

[bsipocz]

I'm not sure why this disappeared with updating the minimum pytest version, but it's gone, and as it was a workaround anyway I don't investigate further.