Install PyMySQL from Git
This is necessary to pull in commit a1ac8239c8 [1], which fixes Python
3.13 compatibility. Prior to this, the tool had in fact been broken
since our commit d949e8ee4a (the initial upgrade to Python 3.13).
Hopefully at some point PyMySQL will put out a new release that includes
the fix and then we can make this a normal version range requirement.