[stubgen] Add required ... rhs to NamedTuple fields with default values

[sobolevn]

Closes #15638

[sobolevn]

After your review, I've noticed that the test case is broken. I was testing it locally with stubgen ex.py && bat out/ex.pyi and it all worked correctly.

For example, stubgen for this file:

# ex.py
from typing import NamedTuple

class A(NamedTuple):
    x: int
    y: str = 'a'

class B(A):
    z1: str
    z2 = 1

class RegularClass:
    x: int
    y: str = 'a'
    class NestedNamedTuple(NamedTuple):
        x: int
        y: str = 'a'

class NamedTupleWithNestedClass(NamedTuple):
    class Nested:
        x: int
        y: str = 'a'

Produces the following .pyi file:

from typing import NamedTuple

class A(NamedTuple):
    x: int
    y: str = ...

class B(A):
    z1: str
    z2: int

class RegularClass:
    x: int
    y: str
    class NestedNamedTuple(NamedTuple):
        x: int
        y: str = ...

class NamedTupleWithNestedClass(NamedTuple): ...

But, here's what is produced with our test case:

from typing import NamedTuple

class A(NamedTuple):
    x: int
    y: str

class B(A):                                   
    z1: str
    z2: int

class RegularClass:
    x: int
    y: str
    class NestedNamedTuple(NamedTuple):
        x: int
        y: str

class NamedTupleWithNestedClass(NamedTuple):
    class Nested:
        x: int
        y: str

Two changes in tests:

1. = ... is missing
2. class Nested: ... is present

What is going on? :)

[sobolevn]

Ok, I need [case testNamedTupleClassSyntax_semanal] :)

[sobolevn]

I've created a new issue about nested classes in NamedTuples: #15752

[AlexWaygood]

LGTM other than my nit about the test