devel/py-ruamel.yaml: update to 0.18.7

Mark as requiring python-3.9+ -- the port itself claims compatibility
all the way back to python-3.7+ but the critical dependency
py-ruamel.yaml.clib does not support the older versions of python.

Changes: https://sourceforge.net/p/ruamel-yaml/code/ci/0.18.7/tree/CHANGES
Reported by: portscout, repology