ValueError: time data '2019-07-18T06:43:39Z' does not match format '%Y-%m-%dT%H:%M:%S.%fZ'

We had a brief burst of these a couple of hours ago. It looks like the fractional seconds part of the timestamp is missing from the `"timestamp"` field in the message, which the parser is expecting:

Sentry Issue: [H-1QJ](https://sentry.io/organizations/hypothesis/issues/1116402353/?referrer=github_integration)

```
ValueError: time data '2019-07-18T06:43:39Z' does not match format '%Y-%m-%dT%H:%M:%S.%fZ'
(19 additional frame(s) were not displayed)
...
File "kombu/messaging.py", line 590, in
[callback(body, message) for callback in callbacks]
File "h/realtime.py", line 53, in handle_message
self._record_time_in_queue(message)
File "h/realtime.py", line 71, in _record_time_in_queue
message.headers["timestamp"], "%Y-%m-%dT%H:%M:%S.%fZ"
File "_strptime.py", line 565, in _strptime_datetime
tt, fraction = _strptime(data_string, format)
File "_strptime.py", line 362, in _strptime
(data_string, format))

Unexpected exception in streamer greenlet:
```