Install PyMySQL from Git
This is necessary to pull in commit a1ac8239c8 [1], which fixes Python
3.13 compatibility. (See also T381923#10850835.) Without this, we can’t
run the tool under Python 3.13, even though we don’t actually use
pymysql at all.
Hopefully at some point PyMySQL will put out a new release that includes
the fix and then we can remove it from our requirements.txt again.