Synapse fails to talk to appservices that listen on an IPv6 address.

This issue has been migrated from [#4092](https://github.com/matrix-org/synapse/issues/4092).

---

### Description

Synapse fails to talk to appservices that listen on an IPv6 address.

### Steps to reproduce

- Set up some appservice
- Use the IPv4 address to verify it's working
- Now change the registration file pointing to the appservice's IPv6
- The logs will now contain errors like this:

```
2018-10-24 16:44:23,723 - synapse.appservice.scheduler - 240 - INFO - - Retrying transaction 338847 for AS ID telegram
2018-10-24 16:44:23,725 - synapse.http.client - 96 - INFO - - Sending request PUT http://[::1]:8187/transactions/338847?access_token=
2018-10-24 16:44:23,726 - synapse.http.client - 118 - INFO - - Error sending request to PUT http://[::1]:8187/transactions/338847?access_token=: IDNAError The label b'::1' is not a valid A-label
2018-10-24 16:44:23,727 - synapse.appservice.api - 249 - WARNING - - push_bulk to http://[::1]:8187/transactions/338847 threw exception The label b'::1' is not a valid A-label
```

Maybe there is more info on this [here](https://matrix.to/#/!HsxjoYRFsDtWBgDQPh:matrix.org/$1540399349358CjNSh:sw1v.org), but I tried to get all the relevant things into this issue.

### Version information

- **Homeserver**: jcg.re
- **Version**: v0.33.7
- **Install method**: docker
- **Platform**: Fedora Host, official docker image, running with network_mode=host in docker-compose