[mypyc] Support __index__ in native classes

[rheard]

Native classes could define __index__, but it wasn't connected to the nb_index slot, so it only worked when called directly. Everything that goes through the slot raised TypeError, including operator.index(), range(), list and str indexing, slicing, hex(), sequence repetition, and the int() fallback for classes without __int__:

class C:
    def __index__(self) -> int:
        return 2

[10, 20, 30][C()]  # TypeError: list indices must be integers or slices, not C

This adds __index__ to the number slots next to __int__ and __float__, and removes it from the list of unsupported dunders in the docs.

Compiled for i in range(c) loops still failed for a different reason, which also affects objects that aren't native. That's fixed separately in #22101 .