subclasses of `datetime` return `datetime` rather than `Self` while time is mocked

### Python Version

3.12.2

### pytest Version

8.1.1

### Package Version

2.14.1

### Description

This code runs fine:
```python
class MyDT(datetime):
    pass

my_dt = MyDT.now()
assert type(my_dt) == MyDT
```

This code asserts:
```python
class MyDT(datetime):
    pass

with time_machine.travel(0):
    my_dt = MyDT.now()
assert type(my_dt) == MyDT
```
