7.1.x doc tweaks to support Sphinx 3.x (#7985)

* This adds a Pipfile for the docs Python package requirements and
  configures it to use Sphinx 3.x.
* Shinx 3.x also requested us to run 2to3 on conf.py in anticipation of
  Sphinx 4.x
* Addressed conf.py processing error concerning
  sphinx.domains.c.CObject.stopwords via nitpick_ignore.
* A couple tweaks were needed to the traffic-server.py for a change to
  the Sphinx  interface (`l_` no longer exists).
3 files changed