Fix Python 3.5 test failure

datasette/app.py

@@ -843,7 +843,7 @@ class Datasette:
     def app_css_hash(self):
         if not hasattr(self, '_app_css_hash'):
             self._app_css_hash = hashlib.sha1(
-                open(os.path.join(app_root, 'datasette/static/app.css')).read().encode('utf8')
+                open(os.path.join(str(app_root), 'datasette/static/app.css')).read().encode('utf8')
             ).hexdigest()[:6]
         return self._app_css_hash