Replace deprecated datetime.utcnow() with timezone-aware equivalent

[mpasternak]

Summary

datetime.datetime.utcnow() has been deprecated since Python 3.12 and is scheduled for removal in a future Python version:

DeprecationWarning: datetime.datetime.utcnow() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to represent datetimes in UTC: datetime.datetime.now(datetime.UTC).

The only call site in the codebase is XMLTreeServer._outputBasicEnvelope in src/oaipmh/server.py, which stamps the <responseDate> element of every OAI-PMH response.

Change

# Before
datetime.utcnow().replace(microsecond=0)

# After
datetime.now(timezone.utc).replace(microsecond=0, tzinfo=None)

datetime_to_datestamp() in oaipmh/datestamp.py asserts dt.tzinfo is None (only accepts naive datetimes), so after obtaining a timezone-aware UTC value we strip tzinfo back off. The resulting responseDate string in the OAI-PMH response is byte-for-byte identical to what the old code produced.

Why this matters

Downstream projects running pytest with filterwarnings = error (or -W error) fail with a 500 on every OAI-PMH verb because the DeprecationWarning is promoted to an exception during response rendering. The change is trivial and semantically transparent.

Test plan

• Manual smoke-test: datetime_to_datestamp(datetime.now(timezone.utc).replace(microsecond=0, tzinfo=None)) produces the same YYYY-MM-DDTHH:MM:SSZ format as before, with no DeprecationWarning on Python 3.13.
• CI runs existing src/oaipmh/tests/ on a supported Python version. (Note: on Python 3.12+ the existing tests fail at collection with ImportError: cannot import name 'makeSuite' from 'unittest' — an unrelated issue since unittest.makeSuite was removed in 3.12.)

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