patman: Drop Python 2 StringIO code

We can rely on Python 3 now, so drop the workaround for importing
StringIO.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/patman/settings.py b/tools/patman/settings.py
index 9ac96c4..21e8fc4 100644
--- a/tools/patman/settings.py
+++ b/tools/patman/settings.py
@@ -34,10 +34,7 @@
     - Merge general default settings/aliases with project-specific ones.
 
     # Sample config used for tests below...
-    >>> try:
-    ...     from StringIO import StringIO
-    ... except ImportError:
-    ...     from io import StringIO
+    >>> from io import StringIO
     >>> sample_config = '''
     ... [alias]
     ... me: Peter P. <likesspiders@example.com>