Make required numpy version for setup and install explicit. (#3163)

* Make required numpy version explicit.

* Read setup requirements from requirements.txt

python/requirements.txt

 six
 numpy==1.15.4; python_version == "3.4"
-numpy; python_version != "3.4"
+numpy==1.16.4; python_version == "2.7"
+numpy; python_version != "2.7" and python_version != "3.4"
 typing

python/setup.py

@@ -369,6 +369,9 @@ class BuildExt(build_ext):
 
 with open(os.path.join(PYNGRAPH_ROOT_DIR, 'requirements.txt')) as req:
     requirements = req.read().splitlines()
+    setup_requires = [
+        item for item in requirements if item.strip().startswith('numpy')
+    ]
 
 setup(
     name='ngraph-core',
@@ -385,7 +388,7 @@ setup(
     packages=packages,
     cmdclass={'build_ext': BuildExt},
     data_files=data_files,
-    setup_requires=['numpy==1.16.4'],
+    setup_requires=setup_requires,
     install_requires=requirements,
     zip_safe=False,
     extras_require={