add urllib3 version restriction to requirements.txt

[rkooo567]

Details are in #678. Once requests library starts to be compatible with urllib3 >= 1.25, we should remove this line.

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[rkooo567]

I added urllib3 constraint to Dev35Dockerfile as well. And the previous CI shows that urllib3==1.25.2 does not generate failed to parse error. I think the problem is within urllib3==1.25.1.

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[simon-mo]

I'm re-running travis. Once it pass we should be fine.