Add a test to select Python 3 via runtime.txt
ADD
tests/venv/py3/README.rst
+System - Specifying runtime environments
+----------------------------------------
+
+You can specify runtime environments (such as Python 2 or 3) with a
+``runtime.txt`` file.
+
+This is an example that selects Python 3. Currently you can not use
+this to select a sepcific version of Python 3 (e.g. 3.4 vs 3.6). If you
+need this level of control we recommend you use a `environment.yml`.

ADD
tests/venv/py3/runtime.txt
+python-3

ADD
tests/venv/py3/verify
+#!/usr/bin/env python3
+import sys
+
+print(sys.version_info)
+assert sys.version_info[:1] == (3,)

Add a test to select Python 3 via runtime.txt