admin/build-doc: use python3

admin/build-doc: use python3

to address https://github.com/sphinx-doc/sphinx/issues/3620, we need to
use sphinx with its fix at
https://github.com/sphinx-doc/sphinx/commit/e049f86b2de1cfdf8a74c88dc9593d047c85d5cb
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>