Update sphinx lower-constraint to match test-requirements
Update sphinx lower-constraint to match test-requirements

The lower-constraint for sphinx conflicts with the range in
test-requirements, but since nothing has changed the requirements files
the check-requirements job hadn't gotten run and it didn't get caught.

Set the lower constraint to the lower bound of the range.

Change-Id: I7f7b15193993542d2c8949a0f99ae86a34753035

dtroyer/osc-lib