specify the highest numpy version under python 2.x (#19018)

As mentioned in this link, the last version of NumPy to support Python 2.7 is numpy 1.16.4.

python/requirements.txt

 requests>=2.20.0
-numpy>=1.12
+numpy>=1.12, <=1.16.4 ; python_version<"3.5"
+numpy>=1.12 ; python_version>="3.5"
 protobuf>=3.1.0
 recordio>=0.1.0
 matplotlib<=2.2.4 ; python_version<"3.6"