Don't let PyInstanceMethod hide itself

Python 3's `PyInstanceMethod_Type` hides itself via its `tp_descr_get`,
which prevents aliasing methods via `cls.attr("m2") = cls.attr("m1")`:
instead the `tp_descr_get` returns a plain function, when called on a
class, or a `PyMethod`, when called on an instance.  Override that
behaviour for pybind11 types with a special bypass for
`PyInstanceMethod_Types`.