bpo-46032: Check types in singledispatch's register() at declaration time (GH-30050)

bpo-46032: Check types in singledispatch's register() at declaration time (GH-30050)

The registry() method of functools.singledispatch() functions checks now
the first argument or the first parameter annotation and raises a TypeError if it is
not supported. Previously unsupported "types" were ignored (e.g. typing.List[int])
or caused an error at calling time (e.g. list[int]).