mysqlclient needs an apt package

[int-y1]

I needed to run sudo apt install pkg-config before I can run pip3 install mysqlclient successfully. Add this command to the docs somewhere.

https://stackoverflow.com/a/76794502

[quantum5]

Thought pkgconf is recommended over pkg-config these days?

[int-y1]

mysqlclient uses pkg-config in the setup instructions

https://github.com/PyMySQL/mysqlclient/blob/e54e8612957e0d74dafe5d186ba1247a612c81bd/README.md?plain=1#L67-L82