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: https://github.com/nodejs/node/pull/29196


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