v0.21.0

• Changed (API): posts.send, and posts.create with send=True, now raise
  on a 403 system_domain_recipient_restricted (with restriction) when the
  team has no verified sending domain and the audience includes anyone outside
  the team, or the post sends from the built-in *.mailtea.email address.
  Nothing is sent or scheduled and the post stays a draft; on posts.create
  the error body carries the draft's id. The send used to be accepted and
  fail a moment later. A broadcast (kind="broadcast") is never published to
  the website, however it is sent. No package change: this is the API's
  behaviour once it is deployed.

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Released from mailtea-app/mailtea@68b43c2.