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

```python
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.