### Pull Request Checklist

<!-- Please read CONTRIBUTING.rst before submitting your pull request -->

* [ ] Pull request is based on the develop branch
* [ ] Pull request includes a [changelog file](https://github.com/matrix-org/synapse/blob/master/CONTRIBUTING.rst#changelog)
* [ ] Pull request includes a [sign off](https://github.com/matrix-org/synapse/blob/master/CONTRIBUTING.rst#sign-off)
