support rST `>>>` (doctest) block highlighting

[flying-sheep]

In reStructuredText documents, using plain >>> syntax creates a “doctest block” with the syntax both GitHub and Pygments call pycon, yet they’re rendered as a plain <pre> block.

This should be fixed. Maybe we should even do what Sphinx does and exclude the >>> and ... line prefixes from selection so people can select/copy only the code!