Upgrade datasette publish Heroku runtime

[simonw]

Python has released a security update! Please consider upgrading to python-3.6.9

https://devcenter.heroku.com/articles/python-support#supported-runtimes shows 3.8.0 is now supported.

[simonw]

datasette/datasette/publish/heroku.py

Line 167 in 973f8f1

open("runtime.txt", "w").write("python-3.6.8")

[simonw]

Here's an instance deployed using the new code which shows Python 3.8.0: https://datasette-fixtures.herokuapp.com/-/versions