Updated documented mysqlclient requirement to 1.3.7.

The test requirement was bumped in
2514e044a7fce58176c4e79eec298f3d8f70e89b.

docs/ref/databases.txt

@@ -335,7 +335,7 @@ Connector/Python includes `its own`_.
 mysqlclient
 ~~~~~~~~~~~
 
-Django requires `mysqlclient`_ 1.3.3 or later.
+Django requires `mysqlclient`_ 1.3.7 or later.
 
 MySQL Connector/Python
 ~~~~~~~~~~~~~~~~~~~~~~