Switch from MySQL-python to PyMySQL

As discussed in the Liberty Design Summit "Moving apps to Python 3"
cross-project workshop, the way forward in the near future is to
switch to the pure-python PyMySQL library as a default.

https://etherpad.openstack.org/p/liberty-cross-project-python3

Change-Id: I590c26523f4dc8d14c45e61bb89555486629b64d

tools/db/schema_diff.py

@@ -32,7 +32,7 @@ Run like:
 
     MYSQL:
 
-    ./tools/db/schema_diff.py mysql://root@localhost \
+    ./tools/db/schema_diff.py mysql+pymysql://root@localhost \
                               master:latest my_branch:82
 
     POSTGRESQL: