MLECO-3962: Set specific versions for Python3.7 in requirements.txt

* For compatibility reasons, use the following versions when
  running on Python3.7:

  numpy 1.21.3
  scipy 1.7.3

Change-Id: I5213820cc64b51cfa8b4ff49a1bff030e652822d
Signed-off-by: Conor Kennedy <conor.kennedy@arm.com>