Exceptions raised while parsing _interpreters configs do not name the offending key

[rajkripal]

Errors from _interpreters.create on a bad config (wrong type, bad string) don't say which key failed, making multi-field configs hard to debug.

Per @picnixz on #148799, add a PEP 678 note via _PyErr_FormatNote at each error site in Python/interpconfig.c.

Related: gh-148798.

[rajkripal]

I plan to pick this up as a small follow-up PR once #148799 is merged, since the most useful note site is the error branch introduced there.