Figure out python 3 support for gomill

[macfergus]

betago uses the gomill library for processing SGF files. Unfortunately, it does not support Python 3. The author appears to have abandoned the project:

mattheww/gomill#1

traveller42 did some work on updating to Python 3, but it seems like there is still a lot left:

https://github.com/traveller42/gomill/tree/python3

So at the moment, betago supports python2 and 3 for playing bots; but only python2 for loading data and training bots.

So we could:

1. Complete the python3 porting work for gomill.
2. Pick out just the SGF reader from gomill and port that bit ourselves.
3. Find another SGF parsing library (I don't see any likely options after a quick search).
4. Drop python 3 support for betago.

I guess I like option 2 the best, and option 4 the least.