Remove the "setuptools" requirement from setup.py
--- a/setup.py
+++ b/setup.py
@@ -37,9 +37,6 @@
 
     # Setuptools metadata.
     zip_safe=False,
-    install_requires=[
-        "setuptools",
-        ],
     test_suite="test.test_enum.suite",
 
     # PyPI metadata.