fix "ModuleNotFoundError: No module named 'pip.req'"
Created by: das-g
See https://stackoverflow.com/a/49867265/674064
The error occurs in our Travis CI builds: https://travis-ci.org/github/geometalab/osmaxx/jobs/694338797#L483
Created by: das-g
See https://stackoverflow.com/a/49867265/674064
The error occurs in our Travis CI builds: https://travis-ci.org/github/geometalab/osmaxx/jobs/694338797#L483