Getting the mysqlclient fix for Python3 to work with Hue's latest release

[Alterrien]

Hi there,

I'd rather post here than create an issue as it's hard to provide a way to reproduce this issue.
For context, my team maintains a fork of Hue to deal with the specific parts of our infra (discovering hiveserver instances from hashicorp's consul, having our own shabby presto connector, etc).
We've been running Hue 4.11 and python 2.7 for some time, and we figure that some bumps are overdue. We've decided to go directly to Python 3.9, and to do that have rebased on the top of cloudera's master branch, and I'm now ironing out all the issues that go with those kind of bumps.

One of the issues I'm encountering is that we need to deal with the mysqlclient swap as documented in the admin documentation. Running through the documented steps does not seem to install the mysqlclient libs. I can build the application, but it fails at startup with a very telling ModuleNotFoundError: No module named 'MySQLdb'
I'm not familiar with python packaging enough to know whether the libs dropped in py-ext are still relevant and used in the hue build, or how to install them with all the changes that have happened since the 4.11 release.

Any help you could provide would be greatly appreciated!
Thanks.