datetime factory functions (utcnow, maybe others) are annotated as Self but return datetime

[tamird]

from datetime import datetime
from typing_extensions import assert_type

class MyDT(datetime):
    pass

my_dt = MyDT.utcnow()
assert_type(my_dt, MyDT)
assert type(my_dt) == MyDT

this type checks, but blows up at runtime. Tested with python 3.11.

EDIT: this issue originally claimed that now() was also incorrectly typed, but it seems that was not correct.

[tamird]

Actually, this whole issue seems to be a bug in time-machine. See adamchainz/time-machine#445.