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.

include/pybind11/pytypes.h

@@ -1183,6 +1183,7 @@ public:
     }
     size_t size() const { return (size_t) PyTuple_Size(m_ptr); }
     detail::tuple_accessor operator[](size_t index) const { return {*this, index}; }
+    detail::item_accessor operator[](handle h) const { return object::operator[](h); }
     detail::tuple_iterator begin() const { return {*this, 0}; }
     detail::tuple_iterator end() const { return {*this, PyTuple_GET_SIZE(m_ptr)}; }
 };
@@ -1220,6 +1221,7 @@ public:
     PYBIND11_OBJECT_DEFAULT(sequence, object, PySequence_Check)
     size_t size() const { return (size_t) PySequence_Size(m_ptr); }
     detail::sequence_accessor operator[](size_t index) const { return {*this, index}; }
+    detail::item_accessor operator[](handle h) const { return object::operator[](h); }
     detail::sequence_iterator begin() const { return {*this, 0}; }
     detail::sequence_iterator end() const { return {*this, PySequence_Size(m_ptr)}; }
 };
@@ -1232,6 +1234,7 @@ public:
     }
     size_t size() const { return (size_t) PyList_Size(m_ptr); }
     detail::list_accessor operator[](size_t index) const { return {*this, index}; }
+    detail::item_accessor operator[](handle h) const { return object::operator[](h); }
     detail::list_iterator begin() const { return {*this, 0}; }
     detail::list_iterator end() const { return {*this, PyList_GET_SIZE(m_ptr)}; }
     template <typename T> void append(T &&val) const {

tests/test_pytypes.cpp

@@ -289,4 +289,8 @@ TEST_SUBMODULE(pytypes, m) {
         l.append(a << b);
         return l;
     });
+
+    m.def("test_list_slicing", [](py::list a) {
+        return a[py::slice(0, -1, 2)];
+    });
 }

tests/test_pytypes.py

@@ -246,3 +246,8 @@ def test_number_protocol():
         li = [a == b, a != b, a < b, a <= b, a > b, a >= b, a + b,
               a - b, a * b, a / b, a | b, a & b, a ^ b, a >> b, a << b]
         assert m.test_number_protocol(a, b) == li
+
+
+def test_list_slicing():
+    li = list(range(100))
+    assert li[::2] == m.test_list_slicing(li)