Remove matplotlib version pin

[kafitzgerald]

PR Summary

Remove matplotlib version pin now that most contour labeling issues have been addressed upstream (e.g. matplotlib/matplotlib#27045)

Also thins the number of contour labels added to the plot_contour_labels usage example to make it look a bit cleaner and remove problematic locations that hit upon the remaining bug in matplotlib contour labeling.

We could opt to leave some of these problematic locations in and wait until there's a fix in matplotlib to remove the version pin, but geocat-examples will still catch this particular issue. Given that this is a bit of a an edge case, I think the benefit of removing the version pin from geocat-viz at this point outweighs the risk to users.

Closes #160

PR Checklist

General

• [x] Make an issue if one doesn't already exist
• [x] Link the issue this PR resolves by adding closes #XXX to the PR description where XXX is the number of the issue.
• [x] Add a brief summary of changes to docs/release-notes.rst in a relevant section for the next unreleased release. Possible sections include: Documentation, New Features, Bug Fixes, Internal Changes, Breaking Changes/Deprecated
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• [x] Make your changes in a forked repository rather than directly in this repo
• [x] Open this PR as a draft if it is not ready for review
• [x] Convert this PR from a draft to a full PR before requesting reviewers
• [x] Passes precommit. To set up on your local, run pre-commit install from the top level of the repository. To manually run pre-commits, use pre-commit run --all-files and re-add any changed files before committing again and pushing.

[kafitzgerald]

Apparently the link for mamba installation instructions changed and that's what the link checker caught.

I'll just update that in here as well.