build: specify Python version once for all tests
Extracted from #28537 for a hopefully shorter review cycle. This makes it easier to experiment with new versions of Python as they become available on the Travis CI platform.
Also: https://blog.travis-ci.com/2019-04-15-xenial-default-build-environment
@nodejs/python
Checklist
-
make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
documentation is changed or added -
commit message follows commit guidelines