Forbid inheritance between PyExt and CyExt

Cython/Compiler/Nodes.py

@@ -4840,6 +4840,12 @@ class CClassDefNode(ClassDefNode):
                      base_type.name in ('tuple', 'str', 'bytes'):
                 error(base.pos, "inheritance from PyVarObject types like '%s' is not currently supported"
                       % base_type.name)
+            elif base_type.is_extension_type and (base_type.nogil != self.nogil):
+                error(base.pos, """
+inheritance between PyExtensionType and CythonExtensionType is not currently supported
+(trying to make %s inherit from %s)
+"""
+                      % (self.class_name, base_type.name))
             else:
                 self.base_type = base_type
             if env.directives.get('freelist', 0) > 0 and base_type != PyrexTypes.py_object_type: