net/py-portend: Update to 2.5

• While I'm here: Add "do-test" target to make future QA easier.

Changelog:

• Fix race condition in "free()" and "occupied()" methods.

PR: 241368
Submitted by: Keith Gaughan <k@stereochro.me> (maintainer)
MFH: 2019Q4 (blanket: reliability fix)