build: specify Python version once for all tests

Extracted from #28537 for shorter review cycle.  This makes it easier to
experiment with new versions of Python as they become available on the
Travis CI platform.

PR-URL: https://github.com/nodejs/node/pull/28694


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>