Use environment markers to define Python2 specific dependencies

The "futures" package need only be installed on Python 2. To specify a
conditional dependency, setuptools, pip, and wheel support "environment
markers". Use this mechanism instead of adding install time logic to the
setup.py file.

Allows restoring universal wheel support, allowing a single wheel for
all version of Python.

For additional details on wheel support for environment markers, see the
documentation at:

https://wheel.readthedocs.io/en/stable/index.html#defining-conditional-dependencies

> Defining conditional dependencies
>
> In wheel, the only way to have conditional dependencies (that might
> only be needed on certain platforms) is to use environment markers as
> defined by PEP 426.
>
> ...
>
> As of setuptools 36.2.1, you can pass extras as part of
> install_requires. The above requirements could thus be written like
> this:
>
> install_requires=[
>     'argparse; python_version=="2.6"',
>     'keyring; extra=="signatures"',
>     'pyxdg; extra=="signatures" and sys_platform!="win32"',
>     'ed25519ll; extra=="faster-signatures"'
> ]