MLBEDSW-6406: Restrict numpy version limit

 - The latest numpy versions require Python 3.8
 - This can cause issues if Python 3.7 is installed which is the version that
Vela is tested against
 - The fix is to limit the numpy version to those that support Python 3.7

Signed-off-by: Tim Hall <tim.hall@arm.com>
Change-Id: I3a388976d5aa76395ca93202e496640c8de9f6f4