Use pure-Python StringIO, not cStringIO

misc.psLib tries to subclass StringIO.  It doesn't work with
the cStringIO version.

Change doesn't seem to affect performance of CFF, which is the
biggest StringIO user.

https://github.com/behdad/fonttools/commit/7279302238a2d57d609cab45934615c4d959c88c#commitcomment-4767054
diff --git a/Lib/fontTools/misc/py23.py b/Lib/fontTools/misc/py23.py
index 2842840..8ff9e65 100644
--- a/Lib/fontTools/misc/py23.py
+++ b/Lib/fontTools/misc/py23.py
@@ -24,7 +24,7 @@
 		return c if isinstance(c, int) else ord(c)
 
 try:
-	from cStringIO import StringIO
+	from StringIO import StringIO
 except ImportError:
 	try:
 		from StringIO import StringIO