Make datetime classmethods return correct type

[PythonCoderAS]

Fix #9578

Confirmed the methods changed do not have the weird behavior as seen in versions prior to Python 3.8 via testing.

[AlexWaygood]

Confirmed the methods changed do not have the weird behavior as seen in versions prior to Python 3.8 via testing.

The change is correct on Python 3.7 also (typeshed still supports 3.7):

(py37) C:\Users\alexw\coding>python
Python 3.7.16 (default, Jan 17 2023, 16:06:28) [MSC v.1916 64 bit (AMD64)] :: Anaconda, Inc. on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import datetime
>>> class Foo(datetime.datetime): ...
...
>>> Foo.combine(datetime.date(1999, 1, 1), datetime.time())
Foo(1999, 1, 1, 0, 0)
>>> type(_)
<class '__main__.Foo'>
>>> Foo.strptime("1999-1-1", "%Y-%m-%d")
Foo(1999, 1, 1, 0, 0)
>>> type(_)
<class '__main__.Foo'>

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

pandas (https://github.com/pandas-dev/pandas)
+ pandas/_libs/tslibs/timestamps.pyi:138: error: Return type "datetime" of "strptime" incompatible with return type "Timestamp" in supertype "datetime"  [override]

pandas-stubs (https://github.com/pandas-dev/pandas-stubs)
+ pandas-stubs/_libs/tslibs/timestamps.pyi:151: error: Return type "datetime" of "strptime" incompatible with return type "Timestamp" in supertype "datetime"  [override]