Temporarily pin Bleach version to <3.0

Bleach is in the middle of being updated to a significant new version
that seems to have quite a few changes (and will probably be a good
update overall that will fix some issues on Tildes), however right now
there are a few things still off with it and it's not safe for me to
update to it yet.

tildes/requirements-to-freeze.txt

@@ -3,7 +3,7 @@ alembic
 amqpy
 argon2_cffi
 black
-bleach
+bleach<3.0  # bleach 3.0.0 has issues, can't upgrade yet
 click
 cornice
 freezegun