We require 3.5 now but we didn't add the metadata. Add it.
While OpenStack broadly only supports 3.6, openstacksdk
has a wider audience, including nodepool which still uses
3.5. So add the 3.5 unit tests back.
Use lower-constraints for py35 unit tests because the
upper-constraints have versions of things that have
droppped py35 support already.
Depends-On: https://review.opendev.org/#/c/715467/
Change-Id: I680e0bab5ba3af7be5e914ece64c6bd25d1f2191
185e55bd77