Travis-CI: Testing with Python 3.7 on three different OSes

What is the minimal .travis.yml file required to run the current Python on Linux, macOS, and Windows?

Ideally, it would be:

language: python
python: 3.7
os:
  - linux
  - osx
  - windows
install: pip3 install --upgrade pip
script: python3 platform-info.py

But what is currently required is: .travis.yml. If any line looks optional, try removing it and see if the tests pass.

Setup times:

• ~15 sec for Python 3.7.1 on Xenial Linux -- dist: xenial
• ~15 sec for Python 3.7.2 on macOS 10.14.3 -- osx_image: xcode10.2
• ~1 min 30 sec Python 3.7.3 on Windows 10.14.3 -- choco install python

As discussed at: https://travis-ci.community/t/python-support-on-windows/241/18

Also see: https://github.com/cclauss/Travis-CI-Python-versions