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