WIP: Python 3 on Linux passes Travis CI tests

[cclauss]

Checklist

• npm install && npm test passes
• tests are included
• documentation is changed or added
• commit message follows commit guidelines

Description of change

The goal was to make our Travis CI tests pass on Python 3.6 on Linux pass. -- Success.

Unfortunately, test/test-find-python.js is so fragile that I can not find a way to do this without breaking ALL Python 2 tests. Hopefully someone else can help to make the Python 2 tests pass again. #1826

I tried without success:

      if (process.env.NODE_GYP_FORCE_PYTHON === '1') {
        t.strictEqual(stderr, '')
        t.ok(/Python 3/.test(stdout)) // Python 3 writes the version to stdout
      } else {
        t.strictEqual(stdout, '')
        t.ok(/Python 2/.test(stderr)) // Python 2 writes the version to stderr
      }

The change to gyp/pylib/gyp/generator/ninja.py is lifted right out of #1836 and should be uncontroversial.

The changes to .travis.yml shift to:

1. focus from Python 3.7 to 3.6 because async and await are reserved words in Python >= 3.7.
2. perform a single test run each on Linux, macOS, and Windows.

[cclauss]

Closing in favor of #1843