test: upgrade Linux Travis CI to Python 3.8

Ensure that we remain compatible with the newest version of Python.

PR-URL: https://github.com/nodejs/node-gyp/pull/1923
Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: João Reis <reis@janeasystems.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>