gh-155245: Fix calendar failing to import when strftime rejects %OB

[tekinertekin]

Fixes #155245.

_localized_month stores the format string and only calls strftime from
__getitem__, so _localized_month('%OB') never raises. The first strftime
call happens further down, in the comparison that detects systems which keep
'%OB' as-is — and that comparison sits in the else branch, outside the
try block meant to catch the failure:

try:
    standalone_month_name = _localized_month('%OB')   # no strftime call yet
    ...
except ValueError:                                    # unreachable
    ...
else:
    if len(set(standalone_month_name)) != ...:        # strftime happens HERE

So on a platform whose strftime rejects %OB, the ValueError escapes and
import calendar fails outright. This is not specific to Wine — as written,
the try/except is unreachable on any such platform.

The fix moves the comparison into the try block so the intended fallback to
month_name/month_abbr applies. No behaviour change where %OB works.

Verified locally on a main build (3.16.0a0) by substituting a datetime.date
subclass whose strftime raises ValueError for %O formats, which reproduces
the reported traceback at calendar.py:156 exactly. The added regression test
uses that substitution plus import_helper.import_fresh_module('calendar'), so
it needs no unusual platform: it fails on unpatched main and passes with the
fix. Full test_calendar passes (86 tests) and Tools/patchcheck is clean.

AI tools were used on this PR: Claude Code (Opus) located the cause, wrote the
patch and the test, and ran the verification. I reviewed the change, understand
it and can explain it, and I take responsibility for it.

• Issue: calendar module fails to import on Wine due to incomplete %OB error handling #155245

[picnixz]

This test isn't really relevant. It would be better to find a locale for which we could check the issue directly without mocking.

[picnixz]

I consider a PR entirely written by an agent as not acceptable. The contribution should be credited to the agent in this case. We do not want to encourage users to use agents just to add a contribution line in their CV. We rather encourage human-driven contributions than automated ones.

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