gh-141540: use %r instead of %s in error messages

[alexwlchan]

I fixed the original example, then looked for other examples of '%s' and "%s" in error messages that might want to be replaced with %r. This isn't all of them, but it was all the cases where I felt confident I understood the original code, the printed value was a str, and this seemed like a sensible suggestion.

I had issues running the tests locally and it's late here, so I'm going to open this PR now and review any CI failures in the morning. 😴

• Issue: strptime: use %r in error messages, not %s #141540

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[serhiy-storchaka]

Some error messages match corresponding error messages in the C code which do not use repr.

Some cases are not so simple. For example the repr of the path on Windows is less readable, and paths on Windows cannot contain newlines, quotes or other control characters. On other hand, if the string which is supposed to represent a path contain weird characters, the repr is more helpful. We use repr in some cases (especially if the path can be bytes) and str in other cases (especially if the path can be Path). Each case should be considered carefully.

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[alexwlchan]

Since I only expanded beyond the initial _strptime.py cases based on a suggestion on the original issue, and I don't care to go through these cases in more detail, I've reduced the scope to just that file. I've also converted all the string formatting in that file to f-strings for consistency.

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