Switch from MySQL-python to PyMySQL
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: I5aa90236424818e9a70f28619b5da5f47330d4e5

openstack/neutron-vpnaas,openstack/neutron-vpnaas