It gets used by pip>=9 to decide which version from pypi to install. Which means we can
more easily drop older Python versions without having to worry about breaking anyones setup.

Sadly our oldest supported system, Ubuntu Xenial, only has pip 8, so it will install
the newest version and fail anyway if we drop Python 3.5 support. We can at least
point users to update their pip in their virtual environments then.

https://packaging.python.org/guides/distributing-packages-using-setuptools/#python-requires

The headers of Python 3.8 trigger the warning and PEP7 states that
Python is depending on this now.

As far as I remember this was mostly added to keep the code compatible
with ancient py2 MSVC, so only use it with Python 2.