change the default to PyMYSQL
change the default 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: Ic609ce136061b753ca692b37509a0b29c60bb8b5

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