runtime.txt – select the Python version

Per https://github.com/heroku/heroku-buildpack-python/issues/318, comments aren’t allowed in runtime.txt, so they’re placed here instead.

See https://devcenter.heroku.com/articles/python-runtimes – this file selects which version of Python Heroku will run with.

runtime.txt contents:

1
python-3.5.2