ctypes: add __ctype_{be,le}__ class attributes

[jorenham]

These class attributes are used when swapping endianness:

>>> import ctypes as ct
>>> ct.c_double.__ctype_le__ is ct.c_double.__ctype_be__
False

There should be need to distinguish between these little- and big-endian type flavors in typeshed, because as far as I can tell, their interfaces are statically equivalent.

[jorenham]

Using a @type_check_only to avoid code duplication seems to confuse mypy, unfortunately.

Also, as noted in python/cpython#148464 (comment), c_longdouble (and c_longdouble_complex) don't have __ctype_{be,le}__ attributes. And unlike c_complex_float and c_complex_double types, it's not clear that these'll gain __ctype_{be,le}__ attributes anytime soon.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉