build: use python3 to build and test on Travis

Request Python 3 with pyenv and ensure that python3 is used by Makefile
to run Python scripts.

PR-URL: https://github.com/nodejs/node/pull/29451


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Rod Vagg <rod@vagg.org>