cairo_backend.py: use BytesIO instead of StringIO
cairo_backend.py: use BytesIO instead of StringIO

This fixes a crash in cairocffi on Python3, and should be compatible
with both python2 and python3. In python2, byte strings are just
strings. In python3, when getting binary data, the user probably wants
a byte string instead of a regular string.

curtacircuitos/pcb-tools