Apply suggestions from code review

Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>

Author: EoinTrial

Doc/library/calendar.rst

@@ -580,11 +580,8 @@ The :mod:`!calendar` module defines the following exceptions:
 
 .. exception:: IllegalMonthError(month)
 
-   A subclass of both :exc:`ValueError` and :exc:`IndexError`,
+   A subclass of :exc:`ValueError` and :exc:`IndexError`,
    raised when the given month number is outside of the range 1-12 (inclusive).
-   The :exc:`IndexError` base class is preserved for backwards compatibility
-   with code that caught :exc:`IndexError` for bad month numbers prior to
-   Python 3.13.
 
    .. versionchanged:: 3.12
       :exc:`IllegalMonthError` is now also a subclass of