Version bump mysqlclient for Django 3.1 compatibility

tox.ini

@@ -30,7 +30,7 @@ deps =
     djmaster: git+https://github.com/django/django.git@master#egg=Django
 
     postgres: psycopg2>=2.6
-    mysql: mysqlclient>=1.3.7,<1.4
+    mysql: mysqlclient>=1.4,<2
     elasticsearch2: elasticsearch>=2,<3
     elasticsearch5: elasticsearch>=5,<6
     elasticsearch5: certifi