python.pkgs.doc8: add pygments dependency

Without pygments doc8 will throw "errors" since it cannot check code
blocks. Pygments has since been added to the dependencies upstream [1],
though that hasn't made it into a release yet. I think we should already
adopt this now.

[1] https://github.com/PyCQA/doc8/commit/b6931816585e224019afddfc9d78ecbcf0549c74