add test case for a C compiler warning due to a bad extension type cast on final method calls

tests/run/inherited_final_method.pyx

+# mode: run
+# tag: exttype, final
+
+cimport cython
+
+
+cdef class BaseClass:
+    """
+    >>> obj = BaseClass()
+    >>> obj.call_base()
+    True
+    """
+    cdef method(self):
+        return True
+
+    def call_base(self):
+        return self.method()
+
+
+@cython.final
+cdef class Child(BaseClass):
+    """
+    >>> obj = Child()
+    >>> obj.call_base()
+    True
+    >>> obj.call_child()
+    True
+    """
+    cdef method(self):
+        return True
+
+    def call_child(self):
+        # original bug: this requires a proper cast for self
+        return self.method()