test-rpi3gm26: Use pip to install nxt-python from GitHub

[gmacario]

You should not have to to "git clone" nxt-python repo, this task may be done directly by "pip install"

See

• Doc: https://pip.pypa.io/en/stable/reference/pip_install/
• Example: https://github.com/wireservice/csvkit/blob/master/requirements-py3.txt

[gmacario]

Fix via https://github.com/SOLARMA/nxt-control-mmf/pull/7