Switch testing from Python 3.5 to 3.7
Switch testing from Python 3.5 to 3.7

Also adds:

- openstack-cover-jobs
- openstack-lower-constraints-jobs
- check-requirements

Change-Id: I609bcc400d00e5b07ac37debe422fb2f37f95d2e

openstack/networking-mlnx,openstack/networking-mlnx,stackforge/networking-mlnx,stackforge/networking-mlnx