Interpret ``ts`` as seconds since the epoch and format as a string according to `RFC3339 <https://www.rfc-editor.org/rfc/rfc3339>`_.
If a second string argument is provided, it is interpreted as a `timezone <https://en.wikipedia.org/wiki/Tz_database>`_ and used to format the timestamp.
..function:: parse_timestamp(str)
Parse ``str`` into seconds since the epoch according to RFC3339.