Use PEP-426 environment markers

Should fix #36

setup.py

@@ -157,7 +157,11 @@ setup(name="ZODB",
       long_description = long_description,
       test_suite="__main__.alltests", # to support "setup.py test"
       tests_require = tests_require,
-      extras_require = dict(test=tests_require),
+      extras_require = {
+        'test': tests_require,
+        'python_version >= "2.7"': 'zodbpickle >= 0.6.0',
+        'platform_python_implementation == "PyPy': 'zodbpickle >= 0.6.0',
+      },
       install_requires = [
         'persistent >= 4.1.0',
         'BTrees >= 4.1.3',
@@ -167,7 +171,7 @@ setup(name="ZODB",
         'zc.lockfile',
         'zdaemon >= 4.0.0a1',
         'zope.interface',
-        ] + (['zodbpickle >= 0.6.0'] if (PY3 or PY27 or PYPY) else []),
+      ],
       zip_safe = False,
       entry_points = """
       [console_scripts]