metadata: work with dicts generated by standard YAML parsers
metadata: work with dicts generated by standard YAML parsers

Now that the mismatch between 'builds' and 'Builds' has been fixed, it is
now possible to read metadata/*.yml files with a standard YAML parser like
PyYAML, then output them using `metadata.write_metadata()`.  Other API
functions in fdroidserver should also work in this case.

I did this as I was working on fdroiddata!7860

f-droid/fdroidserver,f-droid/fdroid-server,f-droid/fdroidserver,fdroidtravis/fdroidserver,f-droid/fdroid-server,f-droid/fdroidserver,f-droid/fdroid-server,f-droid/fdroidserver,f-droid/fdroidserver,fdroidtravis/fdroidserver,f-droid/fdroid-server,fdroidtravis/fdroidserver,f-droid/fdroid-server,fdroidtravis/fdroidserver