Add Python 3.15 calendar datetime and difflib updates

[JelleZijlstra]

Summary

• Add Python 3.15 calendar additions for standalone month names and HTMLCalendar.formatmonthpage.
• Update renamed 3.15 datetime ISO/strptime positional-only parameters, including datetime.fromisoformat.
• Add difflib.unified_diff(color=...).
• Remove corresponding 3.15 stubtest allowlist entries.

Relevant report entries

• Added calendar.standalone_month_name, calendar.standalone_month_abbr, and HTMLCalendar.formatmonthpage() (CPython source: https://github.com/python/cpython/blob/e81025e6d2e01e38c5a5b656af5739ac9df5ff55/Lib/calendar.py)
• Renamed datetime ISO/strptime positional-only parameters to string (CPython source: https://github.com/python/cpython/blob/e81025e6d2e01e38c5a5b656af5739ac9df5ff55/Modules/_datetimemodule.c)
• Added difflib.unified_diff(color=...) (CPython PR: gh-133722: Add Difflib theme to _colorize and 'color' option to difflib.unified_diff cpython#133725; source: https://github.com/python/cpython/blob/main/Lib/difflib.py)

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[srittau]

The situation here is more complicated. There are instances of _localized_month, which isn't a full Sequence, but has other format field. Same actually for month_name and month_abbr. day_name and day_abbr are not instances of _localized_day. But the latter has been true since at least 3.10, so I'm fine with using Sequence in this PR, although I would prepare a followup, changing these fields.

Another thing I noticed, although that's probably legacy and possibly also out of scope: Through __getattr__, this module has two constants "January" (equaling 1), and "February" (equaling 2).

[JelleZijlstra]

Thanks, I'll prepare a followup for this.

Regarding the __getattr__, seems better to keep those out of the stub unless someone really needs them.