This is probably a result of a left over build directory, take a look at /tmp/pip_build_<user> I believe is where this would be. Most likely if you delete that and then re-run the command it'll work. There is an upstream bug here: https://github.com/pypa/pip/issues/1964, which I fixed a week ago but it has not yet been released.
--- Donald Stufft PGP: 7C6B 7C5D 5E2B 6356 A926 F04F 6E3C BCE9 3372 DCFA -- To UNSUBSCRIBE, email to [email protected] with a subject of "unsubscribe". Trouble? Contact [email protected]

