[Stats] Fix cairo error and failing tests

Fixed the Stats GTKUI test not updated to the new GTK3 dir layout.

Fixed pygobject cairo ImageSurface error:

    AttributeError: 'gi.repository.cairo' object has no attribute 'ImageSurface'

The documentation seems to suggest that using `import cairo` is the correct
usage and this fixed the issue, along with adding suggested gi.require_foreign
call.

References:

https://pygobject.readthedocs.io/en/latest/guide/cairo_integration.html

deluge/plugins/Stats/deluge_stats/graph.py

@@ -20,7 +20,11 @@ import logging
 import math
 import time
 
-from gi.repository import cairo
+import gi
+
+gi.require_foreign('cairo')  # NOQA: E402
+
+import cairo  # isort:skip (gi checks required before import).
 
 log = logging.getLogger(__name__)
 

deluge/plugins/Stats/deluge_stats/tests/test_stats.py

@@ -75,14 +75,14 @@ class StatsTestCase(BaseTestCase):
         from deluge_stats import graph, gtkui
 
         from deluge.configmanager import ConfigManager
-        from deluge.ui.gtkui.gtkui import DEFAULT_PREFS
-        from deluge.ui.gtkui.mainwindow import MainWindow
-        from deluge.ui.gtkui.pluginmanager import PluginManager
-        from deluge.ui.gtkui.preferences import Preferences
-        from deluge.ui.gtkui.torrentdetails import TorrentDetails
-        from deluge.ui.gtkui.torrentview import TorrentView
+        from deluge.ui.gtk3.gtkui import DEFAULT_PREFS
+        from deluge.ui.gtk3.mainwindow import MainWindow
+        from deluge.ui.gtk3.pluginmanager import PluginManager
+        from deluge.ui.gtk3.preferences import Preferences
+        from deluge.ui.gtk3.torrentdetails import TorrentDetails
+        from deluge.ui.gtk3.torrentview import TorrentView
 
-        ConfigManager('gtkui.conf', defaults=DEFAULT_PREFS)
+        ConfigManager('gtk3ui.conf', defaults=DEFAULT_PREFS)
 
         self.plugins = PluginManager()
         MainWindow()