E.G. not setting PYTHONPATH in:

 PYTHONPATH=/tmp/r2/lib/python2.7/site-packages python2 ./setup.py install --prefix /tmp/r2

results in:

  running install
  Checking .pth file support in /tmp/r2/lib/python2.7/site-packages/
  /usr/bin/python2 -E -c pass
  TEST FAILED: /tmp/r2/lib/python2.7/site-packages/ does NOT support .pth files
  error: bad install directory or PYTHONPATH

  You are attempting to install a package to a directory that is not
  on PYTHONPATH and which Python does not read ".pth" files from.  The
  ...

The same command under python3 works fine. Is this more setuptools fallout?

Note the end of the error message references easy-install:

  * You can set up the installation directory to support ".pth" files by
    using one of the approaches described here:

  https://setuptools.readthedocs.io/en/latest/easy_install.html#custom-installation-
locations

why is easy-install being invoked??