Mark pymbolic as non-zip-safe.

setup.py

@@ -52,5 +52,7 @@ setup(name="pymbolic",
       install_requires=[
           'pytools>=2'
           ],
-      package_dir={"pymbolic": "src", "pymbolic.mapper":"src/mapper"}
+      package_dir={"pymbolic": "src", "pymbolic.mapper":"src/mapper"},
+
+      zip_safe=False,
      )