Base class not needed in forward extension class declaration

+
+Enhancements:
+
+    - It is no longer necessary to specify the base class of an
+        extension type in a forward declaration. Also, if the class is
+        defined in a .pxd file, the base class only needs to be specified
+        in the .pxd file, not the .pyx file.
+        [Arc Riley]
-

Cython/Compiler/Symtab.py

@@ -912,11 +912,16 @@ class ModuleScope(Scope):
         if entry:
             type = entry.type
             if not (entry.is_type and type.is_extension_type):
-                entry = None # Will cause an error when we redeclare it
+                entry = None # Will cause redeclaration and produce an error
             else:
+                scope = type.scope
+                if typedef_flag and scope.defined:
                     self.check_previous_typedef_flag(entry, typedef_flag, pos)
-                if base_type != type.base_type:
+                if (scope and scope.defined) or (base_type and type.base_type):
+                    if base_type and base_type is not type.base_type:
                         error(pos, "Base type does not match previous declaration")
+                if base_type and not type.base_type:
+                    type.base_type = base_type
         #
         #  Make a new entry if needed
         #