build: add test run Python 3.7 on macOS
build: add test run Python 3.7 on macOS

Refs: https://github.com/nodejs/node-gyp/pull/1846
PR-URL: https://github.com/nodejs/node-gyp/pull/1843
Reviewed-By: João Reis <5095b819f0ac7660978fa44706cfa411a9116ec5@janeasystems.com>

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