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>

tools/patman/func_test.py

@@ -12,10 +12,7 @@ import sys
 import tempfile
 import unittest
 
-try:
-    from StringIO import StringIO
-except ImportError:
-    from io import StringIO
+from io import StringIO
 
 import gitutil
 import patchstream

tools/patman/settings.py

@@ -34,10 +34,7 @@ class _ProjectConfigParser(ConfigParser.SafeConfigParser):
     - 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>

tools/patman/test_util.py

@@ -10,10 +10,7 @@ import sys
 
 import command
 
-try:
-  from StringIO import StringIO
-except ImportError:
-  from io import StringIO
+from io import StringIO
 
 PYTHON = 'python%d' % sys.version_info[0]