Use “… is not …” everywhere instead of “not … is …”.

src/common/jingle_session.py

@@ -527,7 +527,7 @@ class JingleSession:
         # Check if there's already a session with this user:
         if contents[0][0] != 'file':
             for session in self.connection.iter_jingle_sessions(self.peerjid):
-                if not session is self:
+                if session is not self:
                     reason = nbxmpp.Node('reason')
                     alternative_session = reason.setTag('alternative-session')
                     alternative_session.setTagData('sid', session.sid)

src/common/socks5.py

@@ -485,7 +485,7 @@ class Socks5(object):
         for ai in self.ais:
             try:
                 self._sock = socket.socket(*ai[:3])
-                if not self.fingerprint is None:
+                if self.fingerprint is not None:
                     if self.file_props.type_ == 's':
                         remote_jid = gajim.get_jid_without_resource(
                             self.file_props.receiver)

src/plugins/gui.py

@@ -159,7 +159,7 @@ class PluginsWindow(object):
         self.uninstall_plugin_button.set_property('sensitive',
             gajim.PLUGINS_DIRS[1] in plugin.__path__)
         self.configure_plugin_button.set_property(
-            'sensitive', not plugin.config_dialog is None)
+            'sensitive', plugin.config_dialog is not None)
 
     def _clear_installed_plugin_info(self):
         self.plugin_name_label.set_text('')