build: use python3 to build and test on Travis

Request Python 3 with pyenv and ensure that python3 is used by Makefile
to run Python scripts.

PR-URL: #29451
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Rod Vagg <rod@vagg.org>

.travis.yml

@@ -6,13 +6,10 @@ x-ccache-setup-steps: &ccache-setup-steps
 
 os: linux
 language: cpp
-# Currently this file can only support one PYTHON_VERSION.
-# To experiment with Python 3, comment out Python 2.7 and uncomment one of the Python 3 versions.
 env:
   global:
-    - PYTHON_VERSION="2.7.15"
-    # - PYTHON_VERSION="3.6.7"
-    # - PYTHON_VERSION="3.7.1"
+    - PYTHON_VERSION="3.7.1"
+    - PYTHON=python3
 jobs:
   include:
     - stage: "Compile"