lib: Updated __cmp__ method for Python3 functionality

.flake8

@@ -55,9 +55,6 @@ per-file-ignores =
     gui/wxpython/rlisetup/wizard.py: E722
     # Generated file
     gui/wxpython/menustrings.py: E501
-    # F821 undefined name 'cmp'
-    # https://github.com/OSGeo/grass/issues/1809
-    python/grass/pydispatch/saferef.py: F821
     # C wrappers call libgis.G_gisinit before importing other modules.
     # TODO: Is this really needed?
     python/grass/pygrass/vector/__init__.py: E402

python/grass/pydispatch/saferef.py

@@ -167,8 +167,25 @@ def __nonzero__(self):
     def __cmp__(self, other):
         """Compare with another reference"""
         if not isinstance(other, self.__class__):
-            return cmp(self.__class__, type(other))
-        return cmp(self.key, other.key)
+            return (self.__class__.__name__ > other.__class__.__name__) - (
+                self.__class__.__name__ < other.__class__.__name__
+            )
+        return (self.key > other.key) - (self.key < other.key)
+
+    def __eq__(self, other):
+        if not isinstance(other, self.__class__):
+            return False
+        return self.key == other.key
+
+    def __lt__(self, other):
+        if not isinstance(other, self.__class__):
+            return self.__class__.__name__ < other.__class__.__name__
+        return self.key < other.key
+
+    def __gt__(self, other):
+        if not isinstance(other, self.__class__):
+            return self.__class__.__name__ > other.__class__.__name__
+        return self.key > other.key
 
     def __call__(self):
         """Return a strong reference to the bound method