gh-153158: Remove the erroneous width argument of HTMLCalendar.formatmonthpage

[tonghuaroot]

HTMLCalendar.formatmonthpage was added in 3.15 with a width argument copied from formatyearpage. A single month has no width to lay out, and HTMLCalendar.formatmonth's third positional is withyear, so width was silently forwarded as withyear. A falsy value dropped the year from the month heading:

>>> import calendar
>>> b'June 2009' in calendar.HTMLCalendar().formatmonthpage(2009, 6, 0)
False   # withyear=0, year dropped

This removes the parameter. The default output is unchanged. formatmonthpage is new in 3.15, which has not shipped a final release (latest tag v3.15.0b3), so removing it rather than deprecating it is safe; this should be backported to 3.15 only.

Note: dropping the parameter also means formatmonthpage(y, m, 3) now passes 3 to css rather than the (bogus) width. formatmonthpage is currently undocumented in Doc/library/calendar.rst; adding a doc entry is a separate follow-up.

I have left the backport label for a maintainer to assign.

• Issue: calendar.HTMLCalendar.formatmonthpage has a bogus width argument #153158