<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def dt_str_to_posix(dt_str):
 """format str to posix. datetime str is of format %Y-%m-%dT%H:%M:%S.%fZ, e.g. 2013-04-12T00:22:27.978Z. According to ISO 8601, T is a separator between date and time when they are on the same line. Z indicates UTC (zero meridian). A pointer: http://www.cl.cam.ac.uk/~mgk25/iso-time.html This is used to parse LastModified node from GCS's GET bucket XML response. Args: dt_str: A datetime str. Returns: A float of secs from unix epoch. By posix definition, epoch is midnight 1970/1/1 UTC. """

  parsable, _ = dt_str.split('.')
  dt = datetime.datetime.strptime(parsable, _DT_FORMAT)
  return calendar.timegm(dt.utctimetuple())