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