New version for libgd 3

[ChrisBarker-NOAA]

libgd 3 will be making many changes / improvements. It would be nice to get py_gd up and in sync with it:

https://github.com/libgd/libgd/wiki/3.0-Roadmap

[ChrisBarker-NOAA]

well, they don't seem to have gotten very far with 3.0, so .....