Address nightly build failure

[jukent]

It looks like one of our dependencies dropped pytz as a dependency and now it is no longer in the environment causing our nightly build to fail.

Likely unrelated to pytz's release yesterday but some other package's release. Interestingly pytz says, "Note
Projects using Python 3.9 or later should be using the support now included as part of the standard library, and third party packages work with it such as tzdata. pytz offers no advantages beyond backwards compatibility with code written for earlier versions of Python."

Should we consider using Python's built-in zoneinfo instead? @dcamron you added this section 5+ years ago to the datetime chapter, what are your thoughts?

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[jukent]

Interesting this is failing with checks that we didn't catch before:

• Link for "https://pangeo.io/" did not resolve (I'll have to add it to error_rules to ignore because it resolves just fine_
  And all over core/matplotlib.md:
  • Linking "{plot type}" to an implicit heading reference, best practice is to create an explicit reference.
    (See: https://mystmd.org/spec/references#targeting-headers)

I'll address these in a push soon.

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[brian-rose]

Should we consider using Python's built-in zoneinfo instead? @dcamron you added this section 5+ years ago to the datetime chapter, what are your thoughts?

I would say yes! I wasn't aware of this, but I don't think it's good for the code examples in Foundations to be propagating outdated ways of doing things. We don't have any mandate to support backwards compatibility here!

[dcamron]

Yes, we have been using datetime.timezone and datetime.UTC (a shortcut) for a while on Unidata projects. Never thought to come check over here, but I have time to go start a review now, and I can remove the pytz dependency in that PR we should be fine without pytz from there.