devel/py-robotframework-pabot: Update to 0.87

• Switch from PyPI to GitHub for a while to make use of the test suite that isn't available in the sdist via PyPI, yet.

• Add a "do-test" target to make future QA easier.

Changelog:

https://github.com/mkorpela/pabot/compare/0.86...0.87

PR: 240481
Submitted by: vladimir.chukharev@gmail.com (maintainer)
Differential Revision: https://reviews.freebsd.org/D21600