Quickstart "Deploy to Heroku" button in the README fails

[robdeadtech]

"Build app" step fails with

-----> Python app detected
-----> Installing python-3.4.3
-----> Installing pip
-----> Installing requirements with pip
       /app/tmp/buildpacks/779a8bbfbbe7e1b715476c0b23fc63a2103b3e4131eda558669aba8fb5e6e05682419376144189b29beb5dee6d7626b4d3385edb0954bffea6c67d8cf622fd51/bin/steps/pip-install: line 5: /app/.heroku/python/bin/pip: No such file or directory
 !     Push rejected, failed to compile Python app.
 !     Push failed

[robdeadtech]

I also experienced this same on my local when attempting git push heroku master. Changing my runtime.txt to python-3.6.1 got me around this. I'm investigating what else might need to change in order to submit a PR.