Hammer PEP-426 markers until they sort-of work

Any attempts to use platform_python_implementation or
implementation_name result in an "Invalid environment marker" error.

Any attempts to use >= to compare python_version result in an "Invalid
environment marker" error.

setup.py

@@ -159,8 +159,7 @@ def read_file(*path):
       tests_require = tests_require,
       extras_require = {
         'test': tests_require,
-        ':python_version >= "2.7"': 'zodbpickle >= 0.6.0',
-        ':platform_python_implementation == "PyPy': 'zodbpickle >= 0.6.0',
+        ':python_version != "2.6"': 'zodbpickle >= 0.6.0',
       },
       install_requires = [
         'persistent >= 4.1.0',