to address https://github.com/sphinx-doc/sphinx/issues/3620, we need to
use sphinx with its fix at
e049f86b2d
in other words, we need to use sphinx v2.0.0 and up. but sphinx 2.0
requires python >= 3.5, so we have to use python3 for building the
documents.
in this change:
* doc-requirements.txt: install python3 packages on debian derivatives
* build-doc: install python3.6 packages from EPEL7, and use python3
venv for using sphinx2
* doc-requirements.txt: bump up all python packages to latest
stable.
Signed-off-by: Kefu Chai <kchai@redhat.com>