Define constants for month numbers in calendar module

[hugovk (Hugo van Kemenade)]

@Rosuav

Since calendar.January and calendar.February are intended for internal use, should they be deprecated in favour of the new enums?

Searching the top 5k packages, they only show up in jsonargparse:

$ python3 ~/github/misc/cpython/search_pypi_top.py -q . "from calendar import .*(January|February)"
./jsonargparse-4.20.1.tar.gz: jsonargparse-4.20.1/jsonargparse_tests/test_signatures.py: from calendar import Calendar, January  # type: ignore

Time: 0:00:16.076726
Found 1 matching lines in 1 projects

$ python3 ~/github/misc/cpython/search_pypi_top.py -q . "calendar\.(January|February)"
./jsonargparse-4.20.1.tar.gz: jsonargparse-4.20.1/jsonargparse_tests/test_signatures.py: self.assertRaises(ArgumentError, lambda: parser.parse_args(['--cal={"class_path":"calendar.January"}']))
./jsonargparse-4.20.1.tar.gz: jsonargparse-4.20.1/jsonargparse_tests/test_signatures.py: self.assertRaises(ArgumentError, lambda: parser.parse_args(['--cal.help=calendar.January']))

Time: 0:00:15.878868
Found 2 matching lines in 1 projects