Python 3

Python 2 is EOL https://www.python.org/doc/sunset-python-2/

Does this or any fork of gerbmerge support current Python 3?

```
python3 ../../gerbmerge/gerbmerge.py --place-file=board_place.txt board.cfg
  File "../../gerbmerge/gerbmerge.py", line 92
    """
      ^
SyntaxError: invalid syntax
```