Try to fix install failure for xapian on 3.11-dev and newer. distutils.sysconfig.get_config_vars('SO') is returning None on these platforms. Use: sysconfig.get_config_vars('SO') instead. This works with my 3.6 install, so hopefully will work with all of the 3.x releases we have.

from roundup.dist.command.build import build_message_files, check_manifest
from setuptools.command.install_lib import install_lib as base

class install_lib(base):

    def run(self):
        check_manifest()
        build_message_files(self)
        base.run(self)