Display a deprecation warning for `runtime.txt`

[edmorley]

Support for .python-version files has been added to the Python CNB (our next-generation buildpack, currently in preview) in heroku/buildpacks-python#272, and is about to be added to this buildpack (the classic Python buildpack; the one currently used on Heroku) in #932 / #1664.

The .python-version file is superior to runtime.txt in several ways:

• it is commonly supported/used by other tools (unlike runtime.txt, which is Heroku proprietary)
• it supports the major Python version syntax (eg 3.X rather than 3.X.Y), so supports automatic security updates without needing to manually bump the patch version each time.

As such, at some time after #932 has been implemented, we should add a deprecation warning when runtime.txt is used, to gently encourage apps to transition to .python-version files.

We'll continue to support runtime.txt in the classic buildpack, but having it show a deprecation warning will ease the future transition to the Python CNB (which itself will drop support for runtime.txt before GA: heroku/buildpacks-python#275).

GUS-W-16878260.

[edmorley]

Deprecation announcement:
https://devcenter.heroku.com/changelog-items/3141