[omemo] Fix loading of style.css with python 3.5

omemo/plugin.py

@@ -139,7 +139,7 @@ class OmemoPlugin(GajimPlugin):
     def _load_css():
         path = Path(__file__).parent / 'gtk' / 'style.css'
         try:
-            with open(path, "r") as file:
+            with path.open("r") as file:
                 css = file.read()
         except Exception as exc:
             log.error('Error loading css: %s', exc)