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. ✅