Packaging dependency exclusions should be handled using markers

[techalchemy]

Greetings! I was preparing to do some work on this package when I noticed an issue in the definition of some of the packaging dependencies. In order to accommodate modern python packaging practices, python-version specific dependencies should be handled using the markers syntax rather than executing a conditional version check at runtime. This allows for tools (like PyPI's new JSON endpoints) to identify all of the dependencies even if they are not themselves running the relevant python versions.

_python2_requires = [
    'future',  # For backport of surrogateescape
] if sys.version_info < (3,) else []

Becomes

setup(
    ...
    install_requires=[
        "future;python_version<3.0"
    ],
)