relax operator[] for tuples, lists, and sequences

object_api::operator[] has a powerful overload for py::handle that can
accept slices, tuples (for NumPy), etc.

Lists, sequences, and tuples provide their own specialized operator[],
which unfortunately disables this functionality. This is accidental, and
the purpose of this commit is to re-enable the more general behavior.

This commit is tangentially related to the previous one in that it makes
py::handle/py::object et al. behave more like their Python counterparts.