Python 3.5 reached end-of-life on 2020-09-12 and Python 3.9 was released on 2020-10-05.

* Add `language-` prefix to output when syntax highlighting is
  disabled for both codehilite and fenced_code extensions.
* Add `lang_prefix` config option to customize the prefix.
* Add a 'pygments' env to tox which runs the tests with Pygments
  installed. Pygments is locked to a specific version in the env.
* Updated codehilite to accept any Pygments options.
* Refactor fenced code attributes.
   - ID attr is defined on `pre` tag.
    - Add support for attr_list extension, which allows setting arbitrary 
      attributes. 
    - When syntax highlighting is enabled, any pygments options can
       be defined per block in the attr list.
    - For backward compatibility, continue to support `hi_lines` outside
      of an attr_list. That is the only attr other than lang which is allowed
      without the brackets (`{}`) of an attr list. Note that if the brackets
      exist, then everything, including lang and hl_lines, must be within
      them.
* Resolves #775. Resolves #334. Addresses #652.

There are too many random arbitrary failures with this check. We still
want the check to run to ensure we don't introduce any new errors.
However, the random errors on external URLs are frustrating, especcially
when a different URL fails on each run. And then when the URL is checked
manually, it is clearly good.

If the tool provided a flag to limit the check to intenral URLs only (it
doesn't), we wouldn't use that anyway because we do want to know when an
extenral URL becomes invalid. Although, I suppose we could do two runs,
one intenral and one external and only allow the external to fail. But
that will have to wait until such a feature is added to the tool or we
find a better tool.

* The build button/badge in the README now only points to push events so that an unmerged failing PR doesn't cause the build status to show as 'failing.'

* Restrict node setup to checklinks env.