build: test Python 3.6 and 3.7 on Travis CI

PR-URL: https://github.com/nodejs/node/pull/29291


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>