create_superevent and update_superevent are failing if there are more than 6 digits of decimal precision

Sentry Issue: GWCELERY-VT

HTTPError: 400 Client Error: Bad Request for url: https://gracedb-playground.ligo.org/api/superevents/
(8 additional frame(s) were not displayed)
...
  File "requests/sessions.py", line 530, in request
    resp = self.send(prep, **send_kwargs)
  File "requests/sessions.py", line 650, in send
    r = dispatch_hook('response', hooks, r, **kwargs)
  File "requests/hooks.py", line 31, in dispatch_hook
    _hook_data = hook(hook_data, **kwargs)
  File "requests_gracedb/errors.py", line 3, in _hook_raise_errors
    response.raise_for_status()
  File "requests/models.py", line 941, in raise_for_status
    raise HTTPError(http_error_msg, response=self)