test: accept Python 3 in test-find-python.js

Fixes: https://github.com/nodejs/node-gyp/issues/1826
PR-URL: https://github.com/nodejs/node-gyp/pull/1843
Reviewed-By: Christian Clauss <cclauss@me.com>
Reviewed-By: Rod Vagg <r@va.gg>
Reviewed-By: Matt Cowley <me@mattcowley.co.uk>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>