get-pipsi custom Python broken

Followup to https://github.com/mitsuhiko/pipsi/issues/88.

$ curl https://raw.githubusercontent.com/mitsuhiko/pipsi/master/get-pipsi.py | python3
...
Installed pipsi binary in /Users/apikeas/.local/bin
pipsi is now installed.
$ pipsi install saws
Running virtualenv with interpreter /Users/apikeas/.local/venvs/pipsi/bin/python3.6
Using base prefix '/usr/local/Cellar/python3/3.6.1/bin/../Frameworks/Python.framework/Versions/3.6'
New python executable in /Users/apikeas/.local/venvs/saws/bin/python3.6
Also creating executable in /Users/apikeas/.local/venvs/saws/bin/python
ERROR: The executable /Users/apikeas/.local/venvs/saws/bin/python3.6 is not functioning
ERROR: It thinks sys.prefix is '/usr/local/Cellar/python3/3.6.1/Frameworks/Python.framework/Versions/3.6' (should be '/Users/apikeas/.local/venvs/saws')
ERROR: virtualenv is not compatible with this system or executable
Failed to create virtualenv.  Aborting.

If I remove pipsi and retry with python instead of python3, everything works. Both Pythons were installed by Homebrew into /usr/local/bin.