[new-pip] [neutron-server] ubuntu source train fails on networking-hyperv

We are currently running into errors when building train neutron images from source. The installation of `networking-hyperv` fails with:

INFO:kolla.common.utils.neutron-server:ERROR: Cannot install networking-hyperv==5.0.0 and networking-hyperv==5.0.1 because these package versions have conflicting dependencies.
INFO:kolla.common.utils.neutron-server:
INFO:kolla.common.utils.neutron-server:The conflict is caused by:
INFO:kolla.common.utils.neutron-server: networking-hyperv 5.0.1 depends on eventlet!=0.18.3, !=0.20.1, <0.21.0 and >=0.18.2
INFO:kolla.common.utils.neutron-server: networking-hyperv 5.0.0 depends on eventlet!=0.18.3, !=0.20.1, <0.21.0 and >=0.18.2
INFO:kolla.common.utils.neutron-server:To fix this you could try to:
INFO:kolla.common.utils.neutron-server:1. loosen the range of package versions you've specified
INFO:kolla.common.utils.neutron-server:2. remove package versions to allow pip attempt to solve the dependency conflict

How to reproduce:
- install kolla from the stable/train branch
- kolla-build --nocache --type source --openstack-release train --base ubuntu neutron

It seems that the requirement `networking-hyperv>=5.0.0,<6.0.0` (in neutron_server_pip_packages) is not compatible with the constraint `eventlet===0.25.2` (in https://releases.openstack.org/constraints/upper/train), removing the constraint or allowing networking-hyperv>=6.0.0 fixes the issue.