Log Message:
py-ruamel-yaml: updated to 0.17.39

0.17.39:
- update README generation, no code changes

0.17.36:
- fixed issue 480, dumping of a loaded empty flow-style mapping with comment failed
- fixed issue 482, caused by DEFAULT_MAPPING_TAG having changes to being a `Tag()`
  instance, not a string
- updated documentation to use mkdocs