False negative when NamedTuple subclass redefines field in covariant manner

The following currently type checks without error.

```python
# pyright: strict
from typing import NamedTuple

class NT1(NamedTuple):
    number: float

class NT2(NT1):
    number: int

x1 = NT1(1.2)
x2 = NT2(1.2) # Should generate an error.
```

This is related to mypy issue https://github.com/python/mypy/issues/8663.