Restore PY2 for 3rd-party plugins

Restored PY2 to avoid breaking compatibility with plugins that imported
PY2 from common.

Ref: https://bitbucket.org/bendikro/deluge-yarss-plugin/issues/67/deluge-210-removed-all-py2-support

deluge/common.py

@@ -75,6 +75,9 @@ JSON_FORMAT = {'indent': 4, 'sort_keys': True, 'ensure_ascii': False}
 DBUS_FM_ID = 'org.freedesktop.FileManager1'
 DBUS_FM_PATH = '/org/freedesktop/FileManager1'
 
+# Retained for plugin backward compatibility
+PY2 = False
+
 
 def get_version():
     """The program version from the egg metadata.