main/py3-yaml: add dependency on python3

Before this change:

$ dabuild sh
✂
$ sudo apk add --no-progress --quiet py3-yaml
$ python3 -c 'import yaml; print(yaml.__version__)'
sh: python3: not found

After this change:

$ dabuild sh
✂
$ sudo apk add --no-progress --quiet py3-yaml
$ python3 -c 'import yaml; print(yaml.__version__)'
5.3.1