Update tools/pip-requires for prettytable

changes pip-requires from:
    prettytable>=0.6,<0.7
to:
    prettytable>=0.6,<0.8

Change-Id: Ia4e7460525e73474c90cd0c50d3d1243f64f68eb
Authored-by: Chris Krelle <nobodycam@gmail.com>
Merge "Update tools/pip-requires for prettytable"