Py3 compatibility

See merge request !18

This allows to build the error message as expected with both
Python 2 and 3.

Important: when using bytes to generate a string, it is mandatory
to decode it in suitable form otherwise the string ends up containing
"Thing: b'bytes_content'".

In Python 3, network is using bytes.

In Python 3 this module is names queue

In Python 3 it is names queue

Queue has been moved and was not used

matplotlib accepts a StringIO for Python 2 but a wants a BytesIO
with Python 3