Fixes for Matplotlib 3.7

[dopplershift]

Description Of Changes

• Fix test that didn't expect ax[hv]line to trigger unit-based axis labels.
• Correct meteogram_metpy.py to adjust for deprecated/removed matplotlib arguments. Was pretty much unneeded.
• Also fix above for removals pending for Matplotlib 3.8--past code wasn't even actually working correctly. Now this properly sets some spacing of the grid lines using Matplotlib's locators.

This also now modifies the nightly workflow to trigger on having a label applied to the PR, which will allow us to test PRs like this which need to run outside our standard config in order to validate the fix.

Checklist

• Closes Nightly build is failing #2902
• Tests updated

[dopplershift]

I guess an alternative to modifying the existing flow with reporting is to create a separate workflow that triggers on the label and doesn't have a reporting job. Not sure if that's worth the effort, but open to opinions from others.

I also just noticed these jobs are running on Python 3.10, so I'll try kicking these over to 3.11 when I patch this PR up to at least address the lint.

[dopplershift]

So it looks like the current iteration requires the label to be reapplied. Not a huge fan, but not exactly wanting to write a bunch of API calls to avoid it...

[dopplershift]

Ok, matplotlib had a version that showed me the right way to do this.

[dopplershift]

Ok, so refactored so that we have:

• a nightly builds workflow that triggers on cron, manual run, and pushes/prs that modify the workflow file--and opens an issue for any failures
• A PR workflow that runs on PRs, but skips if we haven't added the appropriate label
• Both of these run a "reusable workflow" that runs tests & doc builds using an unstable set of dependencies

[dcamron]

I'm not sure how result propagates, so do

MetPy/.github/workflows/nightly-builds.yml

Lines 56 to 60 in e493f6d

	if ('${{ needs.Tests.result }}' === 'failure') {
	const test_log = fs.readFileSync('tests-nightly.log', 'utf8').substring(0, 21000);
	body += `The tests failed.\nLog:\n<details><pre>${test_log}</pre></details>`;
	}
	if ('${{ needs.Docs.result }}' === 'failure') {

need updated?

[dopplershift]

Good catch. For one, I mistakenly left a condition on even kicking off the builds. Then it's definitely not requesting the results correctly. Probably gonna need some "prints" to figure out the right syntax here, assuming it's available.

[dopplershift]

I think we're good to go here now.