Fix test_session with msgpack-1

[mgorny]

Replace the encoding parameter with raw=False to fix test_session
with msgpack-1.0.0+. The encoding parameter was already deprecated
in msgpack-0.6.2, and raw=False is compatible with that version too.

[davidbrochart]

Thanks @mgorny. In the breaking changes in msgpack 1.0 I can see:

encoding option is removed. You can use raw=True to support old format.

Should we use raw=True?

[mgorny]

That's weird. The old version said:

jupyter_client/tests/test_session.py::TestSession::test_datetimes_msgpack
  /var/tmp/jupyter_client/jupyter_client/tests/test_session.py:307: DeprecationWarning: encoding is deprecated, Use raw=False instead.
    unpack=lambda buf: msgpack.unpackb(buf, encoding='utf8'),

... and this sounds logical to me -- encoded = non-raw.

[mgorny]

Also in the (old) code:

        if typ == TYPE_RAW:
            if self._encoding is not None:
                obj = obj.decode(self._encoding, self._unicode_errors)
            elif self._raw:
                obj = bytes(obj)
            else:
                obj = obj.decode('utf_8')

so raw = False is equivalent to utf-8 encoding.

[davidbrochart]

Yes, they also say:

`raw=False` by default. It assumes str types are valid UTF-8 string and decode them to Python str (unicode) object.

which seems to be what we were expecting previously.
Thanks!