lower numpy version in requirements (#38307)

* test if numpy=1.19.5 meet unittest's requirement

* modify numpy version required

python/requirements.txt

 requests>=2.20.0
-numpy>=1.20 ; python_version>="3.5"
+numpy>=1.13
 protobuf>=3.1.0
 Pillow
 six

python/unittest_py/requirements.txt

@@ -10,3 +10,4 @@ paddle2onnx>=0.8.2
 scipy>=1.6
 prettytable
 distro
+numpy>=1.20