Install osmo-python-tests with pip
Use pip to install osmo-python-tests instead of calling setup.py, so it
works with modern python packaging as well.
Related: osmo-python-tests I186544163ab02834c33f1363f3a06cd380898f56
Change-Id: I9bb5d13897dbf1be9855c4eba29c6b3a2c35729c
