gh-140596: Substitute earlier type parameters into PEP 696 defaults

[KotlinIsland]

A type parameter default may refer to type parameters that appear earlier in
the same type parameter list, but those references were never substituted:

class Bar[T, S = T]: ...

Bar[int].__args__  # (int, ~T)  -- should be (int, int)

Because the unsubstituted T stayed in the specialization, it leaked into
__parameters__, so omitting a defaulted type parameter when subclassing
failed:

class Baz[U](Bar[U]): ...
TypeError: Some type variables (T) are not listed in Generic[U]

PEP 696 and the typing spec both allow a default to refer to a prior type
parameter and require non-overridden defaults to be substituted in.

Fix

The substitution is done in __typing_prepare_subst__, at the point where the
default is filled in. That is the natural place for it: a type parameter only
falls back to its default once every preceding type parameter has a value, so
args already holds exactly the bindings the default needs.

Doing it there means one shared helper (typing._resolve_type_param_default)
covers TypeVar, ParamSpec and TypeVarTuple defaults, and covers both
_generic_class_getitem and _GenericAlias._determine_new_args — the alias
path had the same bug:

T = TypeVar('T')
U = TypeVar('U', default=T)
Union[T, U][int]  # was `int | ~T`, now `int`

TypeVar.__typing_prepare_subst__ is implemented in C, so it calls the helper
through call_typing_func_object, the same way the neighbouring
_typevar_subst, _paramspec_prepare_subst and _typevartuple_prepare_subst
hooks already do. The call only happens when a default is actually used.

Type parameters that come from an enclosing scope (they are not in the
subscripted object's __parameters__) are left untouched, preserving the
existing behaviour for free type variables.

Behaviour change

A default that refers to a type parameter which is not declared before it now
raises TypeError when the default is used, rather than silently producing a
nonsensical specialization. This covers self-references and cycles, both of
which the typing spec disallows:

class A[T = T]: ...
class B[T = S, S = T]: ...

A[()]
TypeError: The default of type parameter T refers to type parameter T, which is not declared before it

[read-the-docs-community]

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