python 3.5 compatibility

basetheme_bootstrap/templatetags/sstatic.py

@@ -22,7 +22,7 @@ def sstatic(context, path):
 
 
 def get_absolut_url(request, relative_url):
-    return f"{request.scheme}://{request.get_host()}{relative_url}"
+    return "%s://%s%s" % (request.scheme, request.get_host(), relative_url)
 
 
 def compute_url_for_path(path):