PEP 747: Minor fixes in code examples (#4193)

- Added missing generic parameter for `strip_annotated_metadata`
- Use the correct `typx` parameter name for `TypeForm` arguments in
  the `split_union` and `as_instance` / `as_type` examples.
- Add `from typing import TypeForm, cast` imports to make the usage
  of `typing` symbols consistent

peps/pep-0747.rst

@@ -410,8 +410,9 @@ extract components of some type form objects.
 ::
 
    import typing
+   from typing import TypeForm, cast
 
-   def strip_annotated_metadata(typx: TypeForm[T]) -> TypeForm[T]:
+   def strip_annotated_metadata[T](typx: TypeForm[T]) -> TypeForm[T]:
        if typing.get_origin(typx) is typing.Annotated:
            typx = cast(TypeForm[T], typing.get_args(typx)[0])
        return typx
@@ -423,15 +424,16 @@ kinds of type form objects:
 
    import types
    import typing
+   from typing import TypeForm, cast
 
    def split_union(typx: TypeForm) -> tuple[TypeForm, ...]:
-       if isinstance(typ, types.UnionType):  # X | Y
-           return cast(tuple[TypeForm, ...], typing.get_args(typ))
-       if typing.get_origin(typ) is typing.Union:  # Union[X, Y]
-           return cast(tuple[TypeForm, ...], typing.get_args(typ))
-       if typ in (typing.Never, typing.NoReturn,):
+       if isinstance(typx, types.UnionType):  # X | Y
+           return cast(tuple[TypeForm, ...], typing.get_args(typx))
+       if typing.get_origin(typx) is typing.Union:  # Union[X, Y]
+           return cast(tuple[TypeForm, ...], typing.get_args(typx))
+       if typx in (typing.Never, typing.NoReturn,):
            return ()
-       return (typ,)
+       return (typx,)
 
 
 Combining with a type variable
@@ -443,7 +445,7 @@ within the same function definition:
 ::
 
    def as_instance[T](typx: TypeForm[T]) -> T | None:
-       return typ() if isinstance(typ, type) else None
+       return typx() if isinstance(typx, type) else None
 
 
 Combining with ``type``
@@ -455,7 +457,7 @@ variable within the same function definition:
 ::
 
    def as_type[T](typx: TypeForm[T]) -> type[T] | None:
-       return typ if isinstance(typ, type) else None
+       return typx if isinstance(typx, type) else None
 
 
 Combining with ``TypeIs`` and ``TypeGuard``