Pin SQLAlchemy to 0.7.x

0.7.10 has been released and is compatible with 0.7.x. Instead
of pinning to 0.7.9, simply pin to 0.7.x to avoid SQLAlchemy 0.8

Unfortuantely '<0.8' matches '0.8.0b2', which is what we don't want.
Therefore pinning it to '<=0.7.99' instead.

This matches a corresponding change in openstack/requirements

Fixes: Bug #1153983

Change-Id: Icf680a933c6092cce52706fd9616a0d208164d1e
(cherry picked from commit 2f2aaf2a89445590e01a7c288de9f5c93b20e87f)