Revert "Do not mark wheel as universal"

[graingert]

Reverts #187

the package already has a valid python_requires:

https://github.com/pytest-dev/pytest-mock/blob/master/setup.py#L11
I think removing the py2 tag from the wheel will cause pip on py2 to download the sdist before seeing the python_requires

[The-Compiler]

What's the point of marking the wheel py2-compatible if it isn't? From https://packaging.python.org/guides/distributing-packages-using-setuptools/#universal-wheels:

Only use the --universal setting, if:

1. Your project runs on Python 2 and 3 with no changes (i.e. it does not require 2to3).
2. Your project does not have any C extensions.

and

Pure Python Wheels that are not “universal” are wheels that are pure Python (i.e. contain no compiled extensions), but don’t natively support both Python 2 and 3.

[graingert]

pypa/packaging.python.org#726