Log Message:
py-ruamel-yaml: updated to 0.16.13

0.16.13:
- fix for issue 359: could not update() CommentedMap with keyword arguments
- fix for issue 365: unable to dump mutated TimeStamp objects
- fix for issue 371: unable to addd comment without starting space
- fix for issue 373: recursive call to walk_tree not preserving all params
- a None value in a flow-style sequence is now dumped as `null` instead
  of `!!null ''`