As part of the push to drop python2 support, replace any explicit python2
dependencies with python3 versions.

For centos, python2 still exists as an implicit dependency, but by adding
python3 we will be able to build even if the configure script begins to
require python 3.5+.

Tested with centos7, fedora, ubuntu, ubuntu1804, and debian 9 (amd64).
Tested under a custom configure script that requires Python 3.5+.

the travis dockerfile is also moved to using python3, which was tested
by running `make docker-test-build@travis`, which I hope is sufficient.
Signed-off-by: NJohn Snow <jsnow@redhat.com>
Message-Id: <20190923181140.7235-7-jsnow@redhat.com>
Signed-off-by: NAlex Bennée <alex.bennee@linaro.org>
Reviewed-by: NEduardo Habkost <ehabkost@redhat.com>
Reviewed-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: NCleber Rosa <crosa@redhat.com>
Tested-by: NCleber Rosa <crosa@redhat.com>