Unsafe subtype: datetime/date

[m-aciek]

Fixes #9015.

This PR introduces unsafe-subtype error code, which makes mypy check fail for unsafe date and datetime classes inheritance, following @JukkaL's comment in the related issue.

Before:

from datetime import date, datetime

if datetime.now() < date.today():  # passing the type checking
    print("that's a surprise!")

After:

from datetime import date, datetime

if datetime.now() < date.today():  # E: Unsupported operand types for < ("datetime" and "date")
    print("that's a surprise!")

This is Copilot agent's work with my prompts' guidance. I reviewed the produced code. The change includes tests and documentation.

Alternatives considered

• remove the inheritance in the typeshed: Experiment: revert datetime-date inheritance typeshed#15136 (worse as it isn't configurable)

Possible addition

• handling of str/Iterable[str] subtyping is straightforward to add

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[sterliakov]

Regarding str vs Iterable[str]: are you sure it's actually a good thing to do? I hate pandas decision to prohibit passing a single string as DataFrame columns, it adds unnecessary friction to quickly creating a one-off dataframe (one has to do columns=list('abc')). Is there a reason to reject passing str as Iterable[str] intentionally? Is it a common source of bugs in some codebase you know/work with?

[sterliakov]

Suggested change

	if self.options and codes.UNSAFE_SUBTYPE in self.options.enabled_error_codes:
	# Block unsafe subtyping relationships when the error code is enabled
	for subclass, superclass in UNSAFE_SUBTYPING_PAIRS:
	if lname == subclass and rname == superclass:
	return False
	if (
	self.options
	and codes.UNSAFE_SUBTYPE in self.options.enabled_error_codes
	and (lname, rname) in UNSAFE_SUBTYPING_PAIRS
	):
	return False

If I'm not mistaken, you don't actually have to iterate manually for this

[sterliakov]

This could be a set, but I'm not sure whether it is actually faster than a single-element list