Avoid attr("__repr__") in initialize_generic (#2317)

If the default argument value is a class, and not an instance of a
class, `a.value.attr("__repr__")` raises a `ValueError`. Switching to
`repr(a.value)` makes this use case work.

Fixes #2028

tests/test_kwargs_and_defaults.cpp

@@ -123,4 +123,9 @@ TEST_SUBMODULE(kwargs_and_defaults, m) {
     py::class_<KWClass>(m, "KWClass")
         .def("foo0", &KWClass::foo)
         .def("foo1", &KWClass::foo, "x"_a, "y"_a);
+
+    // Make sure a class (not an instance) can be used as a default argument.
+    // The return value doesn't matter, only that the module is importable.
+    m.def("class_default_argument", [](py::object a) { return py::repr(a); },
+        "a"_a = py::module::import("decimal").attr("Decimal"));
 }