Your problem may be that the dependency on py-libxml2 is thought to be satisfied by doing an install in a working directory already configured for py37. I use DEPENDS_TARGET= package-install clean because I like not having working directories. That would probably resolve your issue. Arguably this is a bug in pkgsrc, and work directories should be versioned the same way as packages. Or, just 'make clean' in that python dir, and restart the build.
Attachment:
signature.asc
Description: PGP signature