pypirc: stipulate UTF-8 encoding

[woodruffw]

This is a kind of interesting/nuanced thing, and I don't know if this PR is actually the right approach 😅 -- I'm opening it for discussion!

Context: .pypirc was never PEP'd; it was seemingly grandfathered in as a distutils implementation detail that gets respected by twine and flit (and perhaps others?). As a result, there's no stipulation as to what charset a .pypirc should be encoded with.

In practice this is mostly a non-issue: most users will default to UTF-8, or in the worst case they'll use whatever system character encoding Python picks and hopefully their consuming tool will too. However this does unfortunately break down sometimes, like with pypa/twine#1268 recently.

I think the best fix here is just to rip the bandaid off and stipulate that .pypirc files should be UTF-8, especially since upcoming Python versions will use UTF-8 in a blanket fashion by default. I've gone with "should" instead of "must" since twine does make an effort to parse using the system charset, but this is a fallback we'd probably like to eventually remove. I'm not sure what flit does, but "should" also felt appropriate given that it's entirely open-ended at the moment 🙂

Open questions:

• Can we just do this, without any additional standardization procedure? I think the answer is yes, since .pypirc is (1) kind of a legacy component anyways, and (2) never went through a packaging PEP (AFAICT). However, I'd like to hear if others feel otherwise.
• Are there other publishing tools that use this file that this needs to be coordinated with? I know twine and flit for certain, and that uv doesn't (yet?) support it. Ref: Feature Request - uv publish recognizes .pypirc file astral-sh/uv#7676

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