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`.

tests/test_methods_and_attributes.cpp

@@ -163,8 +163,8 @@ public:
 class NotRegistered {};
 
 test_initializer methods_and_attributes([](py::module &m) {
-    py::class_<ExampleMandA>(m, "ExampleMandA")
-        .def(py::init<>())
+    py::class_<ExampleMandA> emna(m, "ExampleMandA");
+    emna.def(py::init<>())
         .def(py::init<int>())
         .def(py::init<const ExampleMandA&>())
         .def("add1", &ExampleMandA::add1)
@@ -222,6 +222,9 @@ test_initializer methods_and_attributes([](py::module &m) {
         .def("__str__", &ExampleMandA::toString)
         .def_readwrite("value", &ExampleMandA::value);
 
+    // Issue #443: can't call copied methods in Python 3
+    emna.attr("add2b") = emna.attr("add2");
+
     py::class_<TestProperties>(m, "TestProperties")
         .def(py::init<>())
         .def_readonly("def_readonly", &TestProperties::value)