test: initial Github Actions with Ubuntu & macOS
test: initial Github Actions with Ubuntu & macOS

Running Python standalone tests on multiple OSes would free up Travis CI for tests of various combinations of Node.js and Python as well as tests on other [CPU architectures](https://docs.travis-ci.com/user/multi-cpu-architectures).  __arch: amd64, arm64, ppc64le, s390x__

PR-URL: https://github.com/nodejs/node-gyp/pull/1985
Reviewed-By: Rod Vagg <f9f03903429036ef609ed7ca822fddfa76155c29@vagg.org>
Reviewed-By: Jiawen Geng <8f5a7089e1809fdab8e8d97ec73a77a8b0770d74@gmail.com>

nodejs/node-gyp,nodejs/node-gyp,nodegit/node-gyp,nodegit/node-gyp,nodejs/node-gyp,nodegit/node-gyp,nodegit/node-gyp,nodejs/node-gyp,nodejs/node-gyp,nodegit/node-gyp