Fix deprecation warning for MsgPack Unpacker encoding

[DavidAntliff]

MsgPack is deprecating Unpacker(encoding='utf-8') - use 'raw=True' as recommended by MsgPack docs: https://pypi.org/project/msgpack/

Deprecating encoding option
encoding and unicode_errors options are deprecated.

In case of packer, use UTF-8 always. Storing other than UTF-8 is not recommended.

For backward compatibility, you can use use_bin_type=False and pack bytes object into msgpack raw type.

In case of unpacker, there is new raw option. It is True by default for backward compatibility, but it is changed to False in near future. You can use raw=False instead of encoding='utf-8'.

Relevant warning:

/home/dantliff/.pyenv/versions/sptest-3.7.1/lib/python3.7/site-packages/zerorpc/events.py:213: PendingDeprecationWarning: encoding is deprecated, Use raw=False instead.
    unpacker = msgpack.Unpacker(encoding='utf-8')