docs: def activate_user(self, username, activation_key):
          """
          Activate an :class:`User` by supplying a valid ``activation_key``.
  
          If the key is valid and an user is found, activates the user and
          return it. Also sends the ``activation_complete`` signal.
  
          :param activation_key:
              String containing the secret SHA1 for a valid activation.
  
          :return:
              The newly activated :class:`User` or ``False`` if not successful.
  
          """
          if SHA1_RE.search(activation_key):
              try:
                  user = self.get(activation_key=activation_key)
              except self.model.DoesNotExist:
                  return False
              if not user.activation_key_expired():
                  user.activation_key = defaults.ACCOUNTS_ACTIVATED
                  user.is_active = True
                  user.save(using=self._db)
                  # Send the activation_complete signal
                  accounts_signals.activation_complete.send(sender=None, 
                      user=user)
                  return user
          return False