Ignore a coverage warning re: sysmon on Pythons < 3.12

[kurtmckee]

Pull Request check-list

Please make sure to review and check all of these items:

• Do tests and lints pass with this change?
• Do the CI tests pass with this change (enable it first in your forked repo and wait for the github action build to finish)?
• Is the new or changed code fully tested?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?
• Is there an example added to the examples folder (if applicable)?
• Was the change added to CHANGES file?

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afterwards / while the PR is open.

Description of change

Coverage currently emits a warning in CI when running on Pythons < 3.12 because COVERAGE_CORE=sysmon regardless of Python version. Recent example on Python 3.8:

../../../../../opt/hostedtoolcache/Python/3.8.18/x64/lib/python3.8/site-packages/coverage/collector.py:156
  /opt/hostedtoolcache/Python/3.8.18/x64/lib/python3.8/site-packages/coverage/collector.py:156: CoverageWarning: sys.monitoring isn't available, using default core (no-sysmon)
    self.warn("sys.monitoring isn't available, using default core", slug="no-sysmon")

This PR configures pytest to ignore the warning.

[kurtmckee]

Rebased on the master branch

[kurtmckee]

Hello @gerzse and @vladvildanov! Please review this PR when you can. It's part of my effort to get CI tests to pass cleanly.