Update requirement for py2 and py3 (#18068)
Update requirement for py2 and py3 (#18068)

for py2:
matplotlib<=2.2.3 ; python_version<"3.5"
scipy>=0.19.0, <=1.2.1 ; python_version<"3.5"
nltk>=3.2.2, <=3.4 ; python_version<"3.5"

for py3:
matplotlib ; python_version>="3.5"
scipy ; python_version>="3.5"
nltk ; python_version>="3.5"

update requests version to >=2.20.0
for CEV-2018-18074

tensor-tang/Paddle,chengduoZH/Paddle,PaddlePaddle/Paddle,luotao1/Paddle,luotao1/Paddle,PaddlePaddle/Paddle,luotao1/Paddle,PaddlePaddle/Paddle,luotao1/Paddle,PaddlePaddle/Paddle,tensor-tang/Paddle,luotao1/Paddle,luotao1/Paddle,chengduoZH/Paddle,chengduoZH/Paddle,tensor-tang/Paddle,luotao1/Paddle,PaddlePaddle/Paddle,tensor-tang/Paddle,chengduoZH/Paddle,PaddlePaddle/Paddle,PaddlePaddle/Paddle,tensor-tang/Paddle,chengduoZH/Paddle