Fix tostring() by joining with bytes instead of str

[dvdkon]

Apparently a leftover from Python 2 to 3 migration, tostring() joined the result of tostringlist() with an empty str instead of an empty bytes. This PR fixes the issue in a way that mirrors the standard library's xml.etree.

[ThomasWaldmann]

@dvdkon thanks for the PR. guess that bug was due to the confusing docstrings/comments. They are from the python2 ages and talk about strings, but mean bytes, not str (python 3 types).

The xml.etree api seems to support encoding="unicode" to support str, but other encodings generate bytes.

AFAICS, emeraldtree does not have that support and only supports encoding to bytes.

[ThomasWaldmann]

@dvdkon just curious, how did you find that?