Upgrade mysqlclient to a supported version

tox.ini

@@ -46,7 +46,7 @@ deps =
     dj21: Django>=2.1,<2.2
 
     postgres: psycopg2>=2.6
-    mysql: mysqlclient==1.3.6
+    mysql: mysqlclient>=1.3.7,<1.4
     elasticsearch2: elasticsearch>=2,<3
     elasticsearch5: elasticsearch>=5,<6
     elasticsearch5: certifi