Alias of a TypedDict not recognized as a TypedDict base

[sterliakov]

Bug Report

Follow-up after #18850 discovery.

When a base TypedDict is defined in sys.version_info branches, mypy does not pick it as a proper TypedDict base.

To Reproduce

from typing import Required, TypedDict

class Base(TypedDict):
    foo: int

BestBase = Base

class Child(BestBase):
    baz: Required[int]  # E: Required[] can be only used in a TypedDict definition

Or playground

Expected Behavior

sys.version_info is a recognized guard, so Child should be treated as regular TypedDict.

Actual Behavior

e.py:9: error: Required[] can be only used in a TypedDict definition  [valid-type]
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 1.15.0 and current master
• Mypy command-line flags: N/A
• Mypy configuration options from mypy.ini (and other config files): N/A
• Python version used: 3.12