cherry pick: use Sequence for default_validators#3

asottile-sentry merged 1 commit intosentry-3.15.1getsentry/sentry-forked-djangorestframework-stubs:sentry-3.15.1from

cherry-pick-fields-sequencegetsentry/sentry-forked-djangorestframework-stubs:cherry-pick-fields-sequenceCopy head branch name to clipboard