[FIX] P3: fix base64 and StringIO uses

* StringIO removed from stdlib, replace with io
* try to correctly handle BytesIO/StringIO (one is for bytes the other
  is for text)
* fix base64: Python 3 removed bytes-encoding and bytes-bytes
  codecs (via #encode) so replace all calls to str.encode('base64'),
  also b64encode is a bytes->bytes conversion so attempt to properly
  handle that

issue #8530