readme_renderer: preserve lang attribute in

 codeblocks

On PyPI, fenced code blocks from markdown are translated to HTML as

<pre>
...
</pre>

On GitHub, the HTML preserves the language attribute (if given),

<pre lang="python">
...
</pre>

This helps third-party applications, like my Purple Pi, to do fancy stuff. (In the case of Purple Pi to render LaTeX math.) I’d love to see this on PyPI, too.