Upgrade some scripts to Python3
Fix no-repeating tweet mechanism typo
Improve backtracking

To be honest, the incidence of those cases is so infinitely small that
I doubt we’d ever notice.
Avoid empty tokens

They’d be skipped as they wouldn’t have mappings, but this makes it
explicit they should not be taken into account.
Reorganize Makefile
Add init-lyrics option

Also ignore lyrics folder in git
Move char removal to post-generation

Having it pre-parse made antirepetition check useless (e.g. generated
tweet had no ¡, but the original tweet may have had them).