Python 3 preparation: StringIO. This generally arranges for StringIO and cStringIO references to use io.StringIO for Python 3 but io.BytesIO for Python 2, consistent with the string representations generally used in Roundup. A special FasterStringIO in the TAL code, which referenced internals of the old Python 2 StringIO module, is cut down so it doesn't actually do anything beyond the StringIO class it inherits from (it would also be reasonable to remove FasterStringIO completely). One place in roundup_server.py clearly needing binary I/O is made to use io.BytesIO unconditionally.