Helps pip decide what version of the library to install.

https://packaging.python.org/tutorials/distributing-packages/#python-requires

> If your project only runs on certain Python versions, setting the
> python_requires argument to the appropriate PEP 440 version specifier
> string will prevent pip from installing the project on other Python
> versions.

https://setuptools.readthedocs.io/en/latest/setuptools.html#new-and-changed-setup-keywords

> python_requires
>
> A string corresponding to a version specifier (as defined in PEP 440)
> for the Python version, used to specify the Requires-Python defined in
> PEP 345.

This was a thing in the now-withdrawn PEP 426.
Fixes #195.

Their presence might give the impression that wheel only works
on CPython and PyPy and not, say, Jython or IronPython.

3.2 its past its End Of Life and 3.3 will be too in September.
They may still continue to work but there is little point in doing extra
work to keep the codebase compatible with these.

Pytest 3.0 deprecated @yield_fixture.