`NamedTuple`s cannot have nested classes

Python allows class definitions inside NamedTuple bodies:

>>> from typing import NamedTuple
>>> class A(NamedTuple):
...     class B: ...
... 
>>> A.B
<class '__main__.A.B'>
>>> A().B
<class '__main__.A.B'>

But, mypy does not allow this:

from typing import NamedTuple

class A(NamedTuple):
    class B: ...  # E: Invalid statement in NamedTuple definition; expected "field_name: field_type [= default]"

reveal_type(A.B)  # E: "type[A]" has no attribute "B"
# N: Revealed type is "Any"

I think that this is a bug that needs to be solved, because we don't have any reason not to allow this (my existing patch shows that semanal and typechecker are happy with allowing these classes).

This is the root cause of https://github.com/python/mypy/issues/15752