Fix caster optimization regression introduced in #3650 (#3659)

* Fix optimization bug introduced in #3650

* Add simple Python extension test for MVF

* Improve comments

* Clarify comment

* Clarify another comment

* Add test docstring

* Fix typo

tests/test_multiple_inheritance.py

@@ -472,3 +472,25 @@ def test_pr3635_diamond_f():
     assert o.get_f_e() == 5
 
     assert o.get_f_f() == 6
+
+
+def test_python_inherit_from_mi():
+    """Tests extending a Python class from a single inheritor of a MI class"""
+
+    class PyMVF(m.MVF):
+        g = 7
+
+        def get_g_g(self):
+            return self.g
+
+    o = PyMVF()
+
+    assert o.b == 1
+    assert o.c == 2
+    assert o.d0 == 3
+    assert o.d1 == 4
+    assert o.e == 5
+    assert o.f == 6
+    assert o.g == 7
+
+    assert o.get_g_g() == 7