Fixes package

[a-tal]

Currently when you try to pull this package in as a dependancy you get an exit 1 error code from pip, due to pulling in futures without it being a hard requirement, as well as the syntax errors below.

Please push a new package to PyPI after accepting this PR

  File "build/bdist.macosx-10.14-x86_64/egg/requests_throttler/example.py", line 35
    print r.response
          ^
SyntaxError: Missing parentheses in call to 'print'. Did you mean print(r.response)?

This backport is meant only for Python 2.
It does not work on Python 3, and Python 3 users do not need it as the concurrent.futures package is available in the standard library.
For projects that work on both Python 2 and 3, the dependency needs to be conditional on the Python version, like so:
extras_require={':python_version == "2.7"': ['futures']}
error: Setup script exited with 1