Merged in...

Merged in agronholm/setuptools/agronholm/remove-the-import-hook-when-unloading-th-1458177009332 (pull request #182)
. Fixes #519.
Remove the import hook when unloading the old pkg_resources modules

ez_setup.py

@@ -192,6 +192,8 @@ def _conflict_bail(VC_err, version):
 
 
 def _unload_pkg_resources():
+    sys.meta_path = [importer for importer in sys.meta_path if
+                     importer.__class__.__module__ != 'pkg_resources.extern']
     del_modules = [
         name for name in sys.modules
         if name.startswith('pkg_resources')