rewrite and extend Caveats

>not supported, since they cannot be represented in the node tree.
It's not that they cannot be represented, it's that they would have to be evaluated.

"inherent limitation in wikicode" sounds misleading it's about generating an AST instead of HTML.

setup.py: Make source files be in a deterministic order

To make the package build reproducibly, sort the source files in a
deterministic order instead of just globbing them.

Patch originally from
<https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=842628>.

Indicate this code is Python 3 only

Clarify due to #156

See https://github.com/ogrisel/python-appveyor-demo/commit/09a1c8672e