Add `datetime.UTC` alias for `datetime.timezone.utc`

[pganssle]

Feature or enhancement

Right now, the UTC singleton is only available as a class-level attribute on datetime.timezone, but I think a lot of people would probably like to be able to do this:

from datetime import UTC

Since datetime.timezone is a class, not a module, right now it is necessary for each person to define their own UTC alias (if they want such a thing) in two lines:

from datetime import timezone
UTC = timezone.utc

This isn't a huge deal, but I think it would be pretty unambiguous what it refers to and might be a nice quality of life improvement.

CC: @abalkin

[pganssle]

I tracked down the decision between datetime.timezone.utc and datetime.UTC to this conversation primarily between @abalkin and @brettcannon. It was definitely a deliberate decision to use timezone.utc, but it doesn't seem like there were especially strong feelings about this.

One counter-example to the "datetime seems to put its attributes in class namespaces" is that MINYEAR and MAXYEAR are global module-level constants. I am mostly convinced that we should go forward with this, particularly since a contributor has already written an excellent PR (#91973) implementing it. Any objections from @brettcannon or @abalkin?

[brettcannon]

Definitely no objections from me. 🙂

[pganssle]

Re-opening because this needs a What's new entry, but I selfishly didn't want to induce a merge conflict with #92177, so I'll add a What's New entry after that one gets merged.