Preserve typevar default None in type alias

[alexdrydew]

Fixes #18188

Currently it seems that None is dropped from type variable defaults when assigning generic class to a type alias.

Example

from typing_extensions import TypeVar
from typing import Generic, Union

T1 = TypeVar("T1", default=Union[int, None])
T2 = TypeVar("T2", default=Union[int, None])


class A(Generic[T1, T2]):
    def __init__(self, a: T1, b: T2) -> None:
        self.a = a
        self.b = b


MyA = A[T1, int]
a: MyA = A(None, 10)  # error: Argument 1 to "A" has incompatible type "None"; expected "int"  [arg-type]
reveal_type(a.a)

The change is similar to #16859

[brianschubert]

mypyc suite failure appears to be due to python/cpython#127307, not related to this PR

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

steam.py (https://github.com/Gobot1234/steam.py)
- steam/ext/commands/cog.py:182: error: Argument 1 to "decorator" has incompatible type "Callable[[Any, VarArg(Any), KwArg(Any)], Coroutine[Any, Any, None]]"; expected "F"  [arg-type]
+ steam/ext/commands/cog.py:182: error: Argument 1 to "decorator" has incompatible type "Callable[[Any | None, VarArg(Any), KwArg(Any)], Coroutine[Any, Any, None]]"; expected "F"  [arg-type]

[hauntsaninja]

Thanks! The mypy logic here seems fragile, we should try setting the default value in the state to True

edit: let's see what #18198 does