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Simplify declaration of install_requires. #9536
The previous way of declaring install_requires is way too overengineered
Don't use environment markers on install_requires as who knows what
Drop the check on Tornado as we don't do anything with it.
(Done in preparation of declaring a dependency on contextlib2 on Py2 to fix #9521 (comment).)
I'm pretty sure the machinery dates back to when dependency installation was a mess and we used to vendor the dependencies, see https://github.com/matplotlib/matplotlib/wiki/MEP11#current-behavior.