Python support for the `.python-version` file

[edmorley]

The Python version for a Python app can currently be configured using a runtime.txt file or for Pipenv users via the Pipfile.lock file.

We should support the .python-version file too, since it's more widely used by other tooling in the Python ecosystem.

See also:

• Python Version via .python-version heroku-buildpack-python#932
• Support flexible patch version syntax to allow for automatic Python updates heroku-buildpack-python#913

[edmorley]

This was released today, see:

• https://github.com/heroku/heroku-buildpack-python/releases/tag/v261
• https://devcenter.heroku.com/changelog-items/3005
• https://devcenter.heroku.com/articles/python-runtimes