make-merge.sh clones cpython branch 3.7

[rffontenelle]

The branch 3.8 is the current stable branch used for translation, but make-merge.sh clones and checkout to 3.7.

While replacing 3.7 -> 3.8 solves it, might be good to think of something more automatic so it isn't necessary to change on every Python release.

[Seluj78]

Hi @rffontenelle ! Yes indeed, make-merge is outdated...

I created that file in hope of having a bot to automatically merge pot files from cpython doc but we thought about it and we prefer to have the make merge done manually

Yet the script still has a reason to live (in this form or another, maybe python idk). The idea being to just launch a script to trigger the make merge and open the PR.