Add TypeForm (PEP 747) to the spec and conformance suite

[JelleZijlstra]

No description provided.

[carljm]

Looks good to me - thank you!

[davidhalter]

Thanks for the quick addition, I think some cases where clarifications would be great (and that are also not really mentioned in the PEP):

def f(x: TypeForm[int | str]): ...

# Are these all fine or not? These are not normal statements and it's quite unusual to
# infer arguments in this way whereas things like `x: TypeAlias = int` are more typical.
# I personally don't care if this is inferred or not, just that it is part of the conformance tests.
f(int)
f(int | str)
f("int")

# There are obviously more complicated cases for type inference like this one:
def g(x: list[TypeForm[int]]): ...
g([int])
g(["int"])
g([TypeForm("int")])  # Obviously fine

This would obviously affect all type inference for all type checkers in quite a big way. Again, I'm just not sure what the PEP wants here, but you can probably clarify. Here I would probably also prefer that at least some of the above cases are added to the conformance tests, because otherwise we would have different behavior for inference in a feature that doesn't really need that. Type inference is already a big mess across type checkers.

[davidhalter]

I would like this case to be clarified with an additional

assert_type(v2_type_form, TypeForm[Any])

since the PEP mentions that this case is not inferred, but Any (which seems reasonable). But I would like this to be consistent across type checkers.

[davidhalter]

Another thing that I just remember is the case of

def takes_type_form(x: TypeForm[int | str]):
a = int | str
b = "int | str"
takes_type_form(a)
takes_type_form(b)

Here I would also like some guidance. Are type checkers expected to store more information than UnionType when inferring int | str.