Upgrade to Python3 (Docker image)

[bzier]

The Docker image is currently using Python2, which has reached EOL. I previously applied a few Python3 compatibility fixes in #52. I have not specifically tested with Python3 in a few years, so it is possible that a few more things may need to be fixed, or it may be as simple as installing Python3 in the image.

[roclark]

FWIW, I have a branch on my fork that updates the base Ubuntu image to a recent Xenial image (newer versions of Ubuntu don't play as nicely with mupen64plus installs I found. I'm sure it will still work, but it was more trouble at the time). In this branch, I compile Python 3.7 inside the container to use for newer RL libraries which appears to work well. So far in all of my testing, I haven't found any of the core gym-mupen64plus code needs to be updated to support Python3 which is great! You can view the update here. If desired, I'd be happy to create a PR for this.

[bzier]

Thanks @roclark! I'll take a look soon. I actually have a TODO list I'm working through, and I have these on the list:

- Create issues:
    - Upgrade base Ubuntu image
    - Explore other base options - e.g. Alpine or other with smaller footprint

I will probably go ahead and create the issues, and will consider your changes under the scope of the first one. As far as Python3 goes, I think it should be a smaller effort to upgrade that in the existing (really, really old) base image. Regardless, even if it ends up being done as part of the same effort, I think the issues can still be independent.

[roclark]

Sounds great! Looking forward to the updates! Thanks again for the incredible work!