Tests require "venv/bin/black"

This fails in a distro build VM, as the distro package "black" will be installed.

I've added `sed -i 's:venv/bin/black:black:' tests/test_skeleton_codestyle.py` to https://build.opensuse.org/package/show/home:jayvdb:py-new/python-plugincode to get the tests passing.

The test here should have a fallback.