remove legacy (python2) StringIO import (#19)

Author: arenevier

tests/utils.py

@@ -25,12 +25,7 @@
 from blockdiag.builder import ScreenNodeBuilder
 from blockdiag.parser import parse_file
 
-try:
-    # sys.stderr in py2.x allows mixture of str and unicode
-    from cStringIO import StringIO
-except ImportError:
-    # sys.stderr in py3.x allows only str objects (disallow bytes objs)
-    from io import StringIO
+from io import StringIO
 
 
 def supported_pil():