python3-ruamel-yaml: upgrade 0.17.20 -> 0.17.21

Changelog:
=========
- fix bug in calling '.compose()' method with 'pathlib.Path' instance.

Signed-off-by: Wang Mingyu <wangmy@fujitsu.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>