Replace deprecated assertNotEquals alias with assertNotEqual

In TestIndex.test_index_mutation

Author: Harmon758

git/test/test_index.py

@@ -595,7 +595,7 @@ def mixed_iterator():
         self._assert_entries(entries)
         self._assert_fprogress(entries)
         self.assertEqual(len(entries), 1)
-        self.assertNotEquals(entries[0].hexsha, null_hex_sha)
+        self.assertNotEqual(entries[0].hexsha, null_hex_sha)
 
         # add symlink
         if not is_win: