Remove checking of PURE_PYTHON at build time.

setup.py

@@ -71,17 +71,19 @@ def _unavailable(self, e):
 py_impl = getattr(platform, 'python_implementation', lambda: None)
 is_pypy = py_impl() == 'PyPy'
 is_jython = 'java' in sys.platform
-is_pure = 'PURE_PYTHON' in os.environ
 
 # Jython cannot build the C optimizations, while on PyPy they are
 # anti-optimizations (the C extension compatibility layer is known-slow,
 # and defeats JIT opportunities).
-if is_pypy or is_jython or is_pure:
+if is_pypy or is_jython:
     ext_modules = []
 else:
     ext_modules = codeoptimization
-tests_require = ['zope.event']
-testing_extras = tests_require + ['nose', 'coverage']
+tests_require = [
+    'coverage',
+    'zope.event',
+]
+testing_extras = tests_require
 
 
 def read(*rnames):