[plistlib] in python3, explicitly use fmt=PlistFormat.FMT_XML
[plistlib] in python3, explicitly use fmt=PlistFormat.FMT_XML

Otherwise it will try to sniff the plist format and call `seek`
method on the file object, but the zip filesystem does not support
that...

UnsupportedOperation: seek

googlefonts/fonttools,fonttools/fonttools