build: add Python 3 tests to Travis CI

These tests are run in allow_failures mode on Python 3.7.1 and they
bypasses the Python version checks in ./configure by directly running
./configure.py.

PR-URL: #29196
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Michaël Zasso <targos@protonmail.com>

.travis.yml

@@ -87,3 +87,18 @@ jobs:
         - if [ "${TRAVIS_PULL_REQUEST}" != "false" ]; then
             bash -x tools/lint-pr-commit-message.sh ${TRAVIS_PULL_REQUEST};
           fi
+
+    - name: "Python 3 is EXPERIMENTAL"
+      language: node_js
+      node_js: "node"
+      install:
+        - pyenv global 3.7.1
+        - python3.7 -m pip install --upgrade pip
+        - make lint-py-build
+      script:
+        - NODE=$(which node) make lint lint-py
+        - python3.7 ./configure.py
+        - NODE=$(which node) make test
+
+  allow_failures:
+    - name: "Python 3 is EXPERIMENTAL"