This updates all Python packages that download their source from PyPi to
use pypi.mk.

This will allow future improvements/changes to pypi.mk to affect all
relevant packages.

This also makes it easier for future Python packages to start using
pypi.mk, when it's clear how it is used in existing packages.

Signed-off-by: Jeffery To <jeffery.to@gmail.com>

The package on PyPi is named `mysqlclient`.
This should have been named `python-mysqlclient` from the start.
There is a `mysql` package on PyPi already but that's a different
code/package.

Doing this should avoid any future confusion.
There is no good time to do this rename; at least 19.07 has been branched
already and this can go into the next release [in a year or so].

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>