Blacklist attr 21.1.0
There is a regression that breaks attr.evolve() when updating attributes that contain an attr class; which we use (eg. for Person or TimestampWithTimezone).
v21.2.0 is expected to fix the issue, but won't be released immediately: https://github.com/python-attrs/attrs/issues/804#issuecomment-833471190
Migrated from D5697 (view on Phabricator)