Revert "Switched to extra_requires for Python 2 specific requirements"

This reverts commit 2e4ab7a6e2.

setup.py

@@ -279,6 +279,9 @@ INSTALL_REQUIRES = (
     'six>=1.5.2',
 )
 
+if not PY3:
+  INSTALL_REQUIRES += ('futures>=2.2.0', 'enum34>=1.0.4')
+
 SETUP_REQUIRES = INSTALL_REQUIRES + (
     'sphinx>=1.3',
     'sphinx_rtd_theme>=0.1.8',
@@ -343,10 +346,4 @@ setuptools.setup(
   install_requires=INSTALL_REQUIRES,
   setup_requires=SETUP_REQUIRES,
   cmdclass=COMMAND_CLASS,
-  extra_requires={
-    ':python_version < "3"': [
-      'futures>=2.2.0',
-      'enum34>=1.0.4'
-    ]
-  },
 )