Issue 46655: typing.TypeAlias is not in the list of allowed plain _SpecialForm typeforms

typing.TypeAlias is allowed to be bare, but it's not listed in the list of types in typing._type_check that are allowed to be bare. This means it's possible to reach the wrong error `TypeError: Plain typing.TypeAlias is not valid as type argument` at runtime.

Examples offhand:
from typing import TypeAlias, get_type_hints
class A:
    a: "TypeAlias" = int
get_type_hints(A)

from typing import Annotated, TypeAlias
b: Annotated[TypeAlias, ""] = int

There's likely more and/or more realistic ways to trigger the problem. Anything that triggers typing._type_check on typing.TypeAlias will give this error (TypeError: Plain typing.TypeAlias is not valid as type argument).

I will fix this by adding TypeAlias to the list of typing special forms allowed to be bare/plain. I intend to move these to their own named var to reduce the chance of types not getting added in the future.

Looks like the more likely use case would be PEP 563:

from __future__ import annotations
from typing import TypeAlias, get_type_hints

import typing

class C:
    a: TypeAlias = int

print(get_type_hints(C))


This prints

Traceback (most recent call last):
  File "C:\Users\gvanrossum\cpython\t.py", line 9, in <module>
    print(get_type_hints(C))
  File "C:\Users\gvanrossum\AppData\Local\Programs\Python\Python310\lib\typing.py", line 1808, in get_type_hints
    value = _eval_type(value, base_globals, base_locals)
  File "C:\Users\gvanrossum\AppData\Local\Programs\Python\Python310\lib\typing.py", line 326, in _eval_type
    return t._evaluate(globalns, localns, recursive_guard)
  File "C:\Users\gvanrossum\AppData\Local\Programs\Python\Python310\lib\typing.py", line 690, in _evaluate
    type_ = _type_check(
  File "C:\Users\gvanrossum\AppData\Local\Programs\Python\Python310\lib\typing.py", line 171, in _type_check
    raise TypeError(f"Plain {arg} is not valid as type argument")