no need to use setuptools_cython anymore

setup.py

@@ -1,18 +1,13 @@
+from __future__ import print_function
 import os
 import re
 import sys
+import pkg_resources
 from glob import glob
 from setuptools import setup, Extension
 from setuptools.command.test import test as TestCommand
 
 
-if sys.version[0] == '2':
-    # setuptools DWIM monkey-patch madness: http://dou.bz/37m3XL
-    if 'setuptools.extension' in sys.modules:
-        m = sys.modules['setuptools.extension']
-        m.Extension.__dict__ = m._Extension.__dict__
-
-
 sources = (glob("src/*.cpp") +
            ["libmc/_client.pyx"])
 include_dirs = ["include"]
@@ -21,6 +16,31 @@
                   "-Wall", "-DMC_USE_SMALL_VECTOR", "-O3", "-DNDEBUG"]
 
 
+def is_installed(requirement):
+    try:
+        pkg_resources.require(requirement)
+    except pkg_resources.ResolutionError:
+        return False
+    else:
+        return True
+
+# Resolving Cython dependency via 'setup_requires' requires setuptools >= 18.0:
+# https://github.com/pypa/setuptools/commit/a811c089a4362c0dc6c4a84b708953dde9ebdbf8
+setuptools_req = "setuptools >= 18.0"
+if not is_installed(setuptools_req):
+    import textwrap
+    print(textwrap.dedent("""
+        setuptools >= 18.0 is required, and the dependency cannot be
+        automatically resolved with the version of setuptools that is
+        currently installed (%s).
+
+        you can upgrade setuptools:
+        $ pip install -U setuptools
+        """ % pkg_resources.get_distribution("setuptools").version),
+        file=sys.stderr)
+    exit(1)
+
+
 def find_version(*file_paths):
     with open(os.path.join(*file_paths)) as fhandler:
         version_file = fhandler.read()
@@ -71,7 +91,7 @@ def run_tests(self):
         "Topic :: Software Development :: Libraries"
     ],
     # Support for the basestring type is new in Cython 0.20.
-    setup_requires=["setuptools_cython", "Cython >= 0.20"],
+    setup_requires=["Cython >= 0.20"],
     cmdclass={"test": PyTest},
     ext_modules=[
         Extension("libmc._client", sources, include_dirs=include_dirs,