uniquify paths in PYTHONPATH

When running in a complex environment with lots of installed
packages, PYTHONPATH gets way too long. Instead, just make sure
that paths_on_pythonpath doesn't contain duplicates

setuptools/command/test.py

@@ -186,11 +186,12 @@ class test(Command):
         orig_pythonpath = os.environ.get('PYTHONPATH', nothing)
         current_pythonpath = os.environ.get('PYTHONPATH', '')
         try:
-            prefix = os.pathsep.join(paths)
-            to_join = filter(None, [prefix, current_pythonpath])
-            new_path = os.pathsep.join(to_join)
-            if new_path:
-                os.environ['PYTHONPATH'] = new_path
+            to_join = []
+            for x in list(paths) + current_pythonpath.split(os.pathsep):
+                if x not in to_join:
+                    to_join.append(x)
+            if to_join:
+                os.environ['PYTHONPATH'] = os.pathsep.join(to_join)
             yield
         finally:
             if orig_pythonpath is nothing: