Add mitogen to the docs requirements
Add mitogen to the docs requirements

Mitogen is dependency of the pyroute2 and was added link to it
the docs in pyroute2 0.6.1. Due to the missing mitogen in our
docs venv, docs job was failing.
So let's install mitogen as docs env requirement, at least for now
to unblock our gate.
Later we can think about better solution as e.g. adding it properly to
the requirements or report and fix the issue in the pyroute2.

Change-Id: Iba37581916842bb5fc9f42f7f483d3431747bf8d
Closes-Bug: #1928913

openstack/neutron,openstack/neutron,mahak/neutron,mahak/neutron,openstack/neutron,mahak/neutron