Add a hook to format Alembic revisions with Black

Alembic 1.2.0 adds the ability to post-process revision scripts, so this
uses that capability to automatically run any new files through Black.
This always had to be done manually before, so it's nice to have it
taken care of automatically like this.

tildes/alembic.ini

@@ -2,6 +2,11 @@
 script_location = alembic
 sqlalchemy.url = postgresql+psycopg2://tildes:@/tildes
 
+[post_write_hooks]
+hooks=black
+black.type=console_scripts
+black.entrypoint=black
+
 # Logging configuration
 [loggers]
 keys = root,sqlalchemy,alembic