Workaround for travis issue:
https://github.com/travis-ci/travis-ci/issues/8363
Sometimes `pip3` works, sometimes not:
pyenv: pip3: command not found
The `pip3' command exists in these Python versions:
3.5
3.5.3
Tried these steps to fix the issue:
- add `python: 3.6` to top level of `.travis.yml`
- add `python3` to `addons.apt.packages` level of `.travis.yml`
- `pyenv global system 3.{4,5,6}`
- `pyenv global 3.6`
In all cases the presence or absence of `pip3` was random.
