Add unit test for cypclass identity checks with mismatched types

tests/run/cypclass_is.pyx

+# mode: run
+# tag: cpp, cpp11
+# cython: experimental_cpp_class_def=True, language_level=2
+
+cdef cypclass LeftBase:
+    pass
+
+cdef cypclass RightBase:
+    pass
+
+cdef cypclass Derived(LeftBase, RightBase):
+    pass
+
+def test_is_cypclass():
+    """
+    >>> test_is_cypclass()
+    (True, True, True, True)
+    """
+    d = Derived()
+    l = <LeftBase> d
+    r = <RightBase> d
+
+    return d is d, l is d, r is d, l is r
+
+def test_is_cypclass_const():
+    """
+    >>> test_is_cypclass_const()
+    (True, True, True, True)
+    """
+    cdef const Derived d = Derived()
+    cdef const LeftBase l = <LeftBase> d
+    cdef const RightBase r = <RightBase> d
+
+    return d is d, l is d, r is d, l is r