admin/build-doc: s/virtualenv/python3 -m venv/

admin/build-doc: s/virtualenv/python3 -m venv/

so we don't need to use virtualenv python package for creating a
virtualenv, the "venv" module in Python3 would suffice.

see also https://docs.python.org/3/library/venv.html

Signed-off-by: Kefu Chai <kchai@redhat.com>
(cherry picked from commit 90c4995c99857e0528922574f1b275ce7fed9d09)

 Conflicts:
	admin/build-doc

Cherry-pick notes:
- admin/build-doc always pip-install's requirements (lacking 144bf1a73
  from master)

Signed-off-by: Ken Dreyer <kdreyer@redhat.com>