Make Python 3 the default lexer for highlighting Python code in Markdown

[theacodes]

Fixes #96

[di]

I'm not sure we want to introduce a shortname (python2) that doesn't actually exist in pygments. I'm also hesitant to add anything that produces inconsistencies between how these READMEs are rendered on PyPI and how they're rendered elsewhere (e.g. on Github), which is what will happen if folks use the python shortname with this PR.

Perhaps the original issue is actually invalid, and we should tell users that want to highlight Python 3 code that they should use the python3 shortname? I agree that python should probably map to the Python 3 lexer someday, but I'm not sure if it's for us to decide.

[theacodes]

Yeah I'm hestant here, but I can think of some arguments in favor of doing this.

The biggest is that GitHub's highlighter defaults python to python3 for example this is just python after the code fence:

async def meep():
   pass

await meep()

Since the goal of supporting gfm is to be as close to github as possible, I am okay with us aliasing python to python3. Especially because without aliasing there's no way to get highlighting to work correctly for Python 3 code on both github and warehouse at the same time.

Whether or not we should allow python2 to work is another story. I wanted at least some sort of escape hatch in case some user really, really needed Python 2 highlighting mode. But, I'm happy to leave that out until someone asks for it.

[di]

Ah, I didn't realize GFM was already mapping python->python3. In that case, we probably should as well.

[di]

Let's ditch the python2 shortname.

[theacodes]

Let's ditch the python2 shortname.

Done.