Update requirements.txt

fix #425 , but we should investigate further

Author: Hagellach37

mapswipe_workers/requirements.txt

@@ -10,5 +10,6 @@ psycopg2-binary==2.8.4
 python-dateutil==2.8.1
 schedule==0.6.0
 sentry-sdk==0.14.0
+six==1.15.0
 slackclient==2.5.0
 xdg==4.0.1