docs: conflict between sphinx-2.1, sphinxcontrib-asyncio

[ploxiln]

Two ways to do this: downgrade sphinx, or get rid of sphinxcontrib-asyncio. I'll push up commits for both.

---

example docs build error:

.../tornado/tornado/auth.py:docstring of tornado.auth.OpenIdMixin.get_authenticated_user:1:Error in "py:comethod" directive:
unknown option: "async".

.. py:comethod:: OpenIdMixin.get_authenticated_user(http_client: tornado.httpclient.AsyncHTTPClient = None) -> Dict[str, Any]
   :module: tornado.auth
   :async:

   Fetches the authenticated user data upon redirect.
   ...

where does that stuff come from? let's see:

$ ag comethod
venv/lib/python3.6/site-packages/sphinxcontrib/asyncio.py
100:class CoMethodDocumenter(MethodDocumenter):
104:    objtype = "comethod"
125:        self.directivetype = "comethod"
149:    app.add_directive_to_domain('py', 'comethod', PyCoroutineMethod)
152:    app.add_autodocumenter(CoMethodDocumenter)
(venv) [pierce@plo-mbp15 tornado]$ ag :async:
venv/lib/python3.6/site-packages/sphinx/ext/autodoc/__init__.py
1060:            self.add_line('   :async:', sourcename)
1362:            self.add_line('   :async:', sourcename)

[ploxiln]

actually, latest sphinx has another issue:

Warning, treated as error:
<unknown>:1:py:class reference target not found: builtins.bytes

(letting it continue to show all such warnings:)

<unknown>:1: WARNING: py:class reference target not found: builtins.bytes
<unknown>:1: WARNING: py:class reference target not found: builtins.str
<unknown>:1: WARNING: py:class reference target not found: builtins.str

[bdarnell]

Thanks!

The issues with both sphinx 2.1 and python 3.8 need to be investigated, but shouldn't block our CI in the meantime.

[bdarnell]

Filed #2677 and #2676 to track the leftover work.