Use environment markers instead of modifying _install_requires

[ianlini]

Fixes #237.

Please see #237 for more information.

[ianlini]

The Windows Py35 pipeline failed when installing enum. I have tried to install my version of nimbusml using Python 3.5.7 with pyenv and ubuntu 18.04. The result is correct and the dependencies shouldn't include enum. I have no idea why the pipeline try to install enum.

[msftclas]

All CLA requirements met.

[ianlini]

After a few hours of investigation, I found that the environment marker syntax I used may be wrong in the Py35 environment. Luckily, after upgrading setuptools, the problem is resolved. It's actually quite usual to upgrade pip, setuptools and wheel before doing anything.

[ianlini]

@ganik Could you take a look at this old PR?

[ianlini]

@pieths Could you take a look at this old PR? Thanks.