build: add test run Node.js 12 & Python 3.7 on macOS

[cclauss]

Checklist

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

Description of change

Travis CI: Add test run Node.js 12 & Python 3.7 on macOS

Edit: This test is currently run in allow_failures mode because Linux and macOS share the same env: values.

[joaocgreis]

@cclauss good call, there are indeed failures there. If you plan to fix these, you can add the fixes to this PR. If not, this new run should be added to allowed failures.

Just in case it's useful, to quickly see the error locally I use

EXPERIMENTAL_NODE_GYP_PYTHON3=1 NODE_GYP_FORCE_PYTHON=python3 node bin/node-gyp.js rebuild -C test/node_modules/hello_world/ --loglevel=verbose

[cclauss]

The Python 3 on macOS job is already run in allow_failures mode because Linux and macOS share the same env: values. The new test in this PR is really only interesting in combination with #1843, #1844, or #1845.

[joaocgreis]

The failures in test-find-python are fixed in #1843, I think they're safe to ignore here. There are other failures there that show real issues.

@cclauss if you'd prefer, I can include this in #1843 leaving osx python 3 in allowed failures. Would that be better?

[cclauss]

Yes. Please include in #1843 and I am OK with leaving osx python 3 in allow_failures mode.

matrix:
  [ ... ]
  allow_failures:
    - name: "Node.js 12 & Python 3.7 on macOS"

[cclauss]

Closing in favor of #1843 which now includes these changes.