With Python 3.6, the PopenProcess class triggers warnings like this: ResourceWarning: subprocess 1234 is still running
The PopenProcess class is fixed in the master branch: https://gitweb.gentoo.org/proj/portage.git/commit/?id=855c8761f85323bbe570ae61e87c57dc8ea5ea0e We've still got some similar ResourceWarning messages triggered by SimpleRepomanTestCase, but that's a separate issue (and debugging it requires lxml built for Python 3.6).
There's a fix for repoman's Status.check method here: https://gitweb.gentoo.org/proj/portage.git/commit/?id=4375ccd4188467dbd15308baf23331668afe4691
Fixed in portage-2.3.5.