Add python_requires setuptools check for python > 3.6

[Nicusor97]

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Description of change

I noticed that in your latest release candidate version 4.0.0b3 the Python 2.7 support has been removed but your forgot to add the python_requires check that will prevent users with Python 2.7 from downloading a sdist version that they cannot build on Python 2.7.

Note: supporting python_requires requires setuptools>=24.2.0 and pip>=9.0.0 to benefit from it.
Details here: https://packaging.python.org/guides/distributing-packages-using-setuptools/#python-requires

I open this MR in order to merge the python_requires before releasing the official version because if the official version will not have this check, the Python 2 users will not be able to build the sdist and in the end that new version should be yanked and release a new version with the python_requires check.

[chayim]

Thanks a tonne! I've transitioned all of my other packages to poetry - and this literally slipped my mind. It's been a few years since I've touched a setup.py

This is an awesome change. Thank you so much! In fact, the more you know about setup.py the better :)