Upgraded the Python 3 warnings

Since some users have ignored the warnings and tried to use this on Python 3, the warning has been changed to a hard failure when running setup.py on py3. The README now also reflects this.

README.rst

diff --git a/README.rst b/README.rst
index 7f1ef68..1305003 100644
--- a/README.rst
+++ b/README.rst
@@ -4,8 +4,9 @@
 
 This is a backport of the `concurrent.futures`_ standard library module to Python 2.
 
-It should not be installed on Python 3, although there should be no harm in doing so, as the
-standard library takes precedence over third party libraries.
+It **does not** work on Python 3 due to Python 2 syntax being used in the codebase.
+Python 3 users should not attempt to install it, since the package is already included in the
+standard library.
 
 To conditionally require this library only on Python 2, you can do this in your ``setup.py``:
 

setup.py

diff --git a/setup.py b/setup.py
index d45e66d..792c4f0 100755
--- a/setup.py
+++ b/setup.py
@@ -1,13 +1,19 @@
 #!/usr/bin/env python
 # coding: utf-8
-from warnings import warn
+from __future__ import print_function
+
 import os.path
 import sys
 
 if sys.version_info[0] > 2:
-    warn('This backport is meant only for Python 2.\n'
-         'Python 3 users do not need it, as the concurrent.futures '
-         'package is available in the standard library.')
+    print('This backport is meant only for Python 2.\n'
+          '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.\n'
+          'For projects that work on both Python 2 and 3, the dependency needs to be conditional '
+          'on the Python version, like so:\n'
+          "extras_require={':python_version == \"2.7\"': ['futures']}",
+          file=sys.stderr)
+    sys.exit(1)
 
 extras = {}
 try: