Simplify setup.py slightly
diff --git a/setup.py b/setup.py
index d8c0790..eb735a0 100644
--- a/setup.py
+++ b/setup.py
@@ -29,22 +29,19 @@
     exec(f.read(), about)
 
 
-SETUPTOOLS_DEPENDENCY = "setuptools"
-CFFI_DEPENDENCY = "cffi>=0.8"
-SIX_DEPENDENCY = "six>=1.4.1"
 VECTORS_DEPENDENCY = "cryptography_vectors=={0}".format(about['__version__'])
 
 requirements = [
     "pyasn1",
-    SIX_DEPENDENCY,
-    SETUPTOOLS_DEPENDENCY
+    "six>=1.4.1",
+    "setuptools"
 ]
 
 if sys.version_info < (3, 4):
     requirements.append("enum34")
 
 if platform.python_implementation() != "PyPy":
-    requirements.append(CFFI_DEPENDENCY)
+    requirements.append("cffi>=0.8")
 
 # If you add a new dep here you probably need to add it in the tox.ini as well
 test_requirements = [