Fix docs reqs for new pip

Per [1], the docs requirements were failing (or at least could) due to
test-requirements being used instead of doc/requirements. The former
may include dep versions not compatible with the upper-constraints and
hence cause issues for the new pip resolver. This patch fixes that.

[1] http://lists.openstack.org/pipermail/openstack-discuss/2021-January/019611.html

Change-Id: Ifcf59e49b00e9b468b59f53de53c417a0e84adc4
Signed-off-by: Nicolas Bock <nicolas.bock@canonical.com>
8 files changed