[stubgen] Nested classes in `NamedTuple`s are not produced

[sobolevn]

Consider this code:

from typing import NamedTuple

class A(NamedTuple):
    class B: ...

In runtime you can freely access it:

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

But, stubgen does ignore it:

» stubgen ex.py && bat out/ex.pyi
Processed 1 modules
Generated out/ex.pyi
───────┬───────────────────────────────────────────────────────────────────────────────────
       │ File: out/ex.pyi
───────┼───────────────────────────────────────────────────────────────────────────────────
   1   │ from typing import NamedTuple
   2   │ 
   3   │ class A(NamedTuple): ...
───────┴───────────────────────────────────────────────────────────────────────────────────

This needs to be fixed.