Add a type parameter for ufunc ntypes property

[eerovaher]

On current main the template for the generic ufunc type-check-only subtypes declares the return type of the ntypes property to be Literal[0] unconditionally. At runtime the value can be 0 or 1 depending on which template is used for creating the ufunc:

pyerfa/erfa/define_ufunc_user_dtype.templ

Line 2 in 343b9c8

NULL, NULL, NULL, 0,

pyerfa/erfa/define_ufunc.templ

Line 2 in 343b9c8

funcs_$pyname, data, types_$pyname, 1,

The code that chooses the template is

pyerfa/erfa_generator.py

Lines 497 to 505 in 343b9c8

	if self.user_dtype:
	placeholders["user_dtype"] = self.user_dtype
	placeholders["register_dtypes"] = "\n".join(
	f"dtypes[{i}] = {arg.dtype};"
	for i, arg in enumerate(self.py_args + self.ufunc_return)
	)
	file = "define_ufunc_user_dtype.templ"
	else:
	file = "define_ufunc.templ"

so the ntypes return type should be a type parameter likewise determined by the user_dtype property of the corresponding Function in erfa_generator.

This change will allow using the ufunc type annotation template for ufuncs with ntypes=1.