Add cftime support for non-standard calendars

[cvanelteren]

This PR introduces support for plotting data with non-standard calendars using cftime.

Key changes:

• Added CFTimeConverter to enable matplotlib to handle cftime.datetime objects.
• Implemented AutoDatetimeLocator and AutoDatetimeFormatter for automatic, "nice" tick generation on cftime axes, mimicking matplotlib's behavior for standard datetime objects.
• Added a regression test using pytest-mpl to ensure visual consistency of plots with non-standard calendars.

This resolves issues where plots with non-standard calendars had incorrect or poorly formatted time axes.

Continuation of #289

[codecov]

Codecov Report

❌ Patch coverage is 77.13816% with 139 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
ultraplot/ticker.py	66.98%	63 Missing and 41 partials ⚠️
ultraplot/tests/test_tickers.py	89.92%	20 Missing and 8 partials ⚠️
ultraplot/internals/rcsetup.py	53.33%	4 Missing and 3 partials ⚠️

📢 Thoughts on this report? Let us know!

[cvanelteren]

Still fixing one test but rest should be ready for review.

[cvanelteren]

We may want to style the formatting further. In particular this

_RESOLUTION_TO_FORMAT = {
    "SECONDLY": "%H:%M:%S",
    "MINUTELY": "%H:%M",
    "HOURLY": "%Y-%m-%d %H:%M",
    "DAILY": "%Y-%m-%d",
    "MONTHLY": "%Y-%m",
    "YEARLY": "%Y",
}

[cvanelteren]

Got the coverage to 65 percent from 0. Little bit done with adding tests and this should be good enough for now.

[Copilot]

Pull Request Overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

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[beckermr]

One more question. Looking good!

[cvanelteren]

Maybe I will add a bit more tests in the next few days but otherwise this is good to go.

[cvanelteren]

Autoformatting prefix and suffix are bugged. Will do this in another PR. I think the coverage is now 78% which is good enough for now. Will wait for codecov confirmation