Disallow str in dates (AnyDate)

[lainets]

gitmanager/access/course.py

Lines 325 to 327 in ba22bed

	AnyDate = Union[datetime, date, str]
	# FIXME: str in AnyDate passes invalid dates and any strings.
	# str should be removed. However, removing it causes crashes with the Undefined type.

should not allow str, as said in the comment. Not sure when the Undefined type issue occurs as removing the str didn't cause any crashes for me.

Invalid dates can be passed to A+ as long as the str exists. Fortunately, the issue is noticed by A+ but the error message will be much less clear when reported by A+ instead of Git Manager.