Support python3

[crazyscot]

(From the discussion in #1)

It would be useful to support python3, seeing as python2 is EOL.
In an ideal world the code might even be polyglotted to support either. (See https://scoutapm.com/blog/automated-tools-and-strategies-to-help-migrate-from-python-2-to-3 ...)

[crazyscot]

OK, the client seems plausibly working locally now using python 3 and wxpython 4, but it is untested against the server and will remain so until I can arrange access to my friend's OB truck or set up a local server myself. If it works for you, please let me know!

[crazyscot]

User report that this mostly works, subject to #9: #1 (comment) refers.

[crazyscot]

References: https://docs.python.org/3/howto/pyporting.html , http://python-future.org/automatic_conversion.html