BF: avoid relpath in test if not needed

relpath giving errors testing on Windows when code not on the
same drive as test.  Avoid using relpath unless the relpath really needs
to be ``..``, in which case test will still error, but as a way of
failing.

Author: matthew-brett

nibabel/externals/tests/test_ordereddict.py

@@ -12,9 +12,9 @@
 
 def test_right_version():
     # If Python < 2.7, use our own copy, else use system copy
-    class_path = inspect.getfile(OrderedDict)
-    rel_dir = dirname(relpath(realpath(class_path), MY_PATH))
     if sys.version_info[:2] < (2, 7):
+        class_path = realpath(inspect.getfile(OrderedDict))
+        rel_dir = dirname(relpath(class_path, MY_PATH))
         assert_equal(rel_dir, '..')
     else:
         import collections