Intersphinx URL for 'requests-oauthlib' now 404s

[tseaver]

From this failed build:

nox > Running session docs
nox > Creating virtual environment (virtualenv) using python3.8 in .nox/docs
nox > python -m pip install -e .[aiohttp]
nox > python -m pip install sphinx alabaster recommonmark sphinx-docstring-typing
nox > sphinx-build -T -W -N -b html -d docs/_build/doctrees/ docs/ docs/_build/html/
Running Sphinx v4.3.0
making output directory... done
loading intersphinx inventory from https://docs.python.org/3.5/objects.inv...
loading intersphinx inventory from https://urllib3.readthedocs.io/en/stable/objects.inv...
loading intersphinx inventory from https://requests.kennethreitz.org/en/master/objects.inv...
loading intersphinx inventory from https://requests-oauthlib.readthedocs.io/en/stable/objects.inv...

Traceback (most recent call last):
  File "/tmpfs/src/github/google-auth-library-python/.nox/docs/lib/python3.8/site-packages/sphinx/cmd/build.py", line 276, in build_main
    app = Sphinx(args.sourcedir, args.confdir, args.outputdir,
  File "/tmpfs/src/github/google-auth-library-python/.nox/docs/lib/python3.8/site-packages/sphinx/application.py", line 270, in __init__
    self._init_builder()
  File "/tmpfs/src/github/google-auth-library-python/.nox/docs/lib/python3.8/site-packages/sphinx/application.py", line 329, in _init_builder
    self.events.emit('builder-inited')
  File "/tmpfs/src/github/google-auth-library-python/.nox/docs/lib/python3.8/site-packages/sphinx/events.py", line 101, in emit
    results.append(listener.handler(self.app, *args))
  File "/tmpfs/src/github/google-auth-library-python/.nox/docs/lib/python3.8/site-packages/sphinx/ext/intersphinx.py", line 240, in load_mappings
    updated = [f.result() for f in concurrent.futures.as_completed(futures)]
  File "/tmpfs/src/github/google-auth-library-python/.nox/docs/lib/python3.8/site-packages/sphinx/ext/intersphinx.py", line 240, in <listcomp>
    updated = [f.result() for f in concurrent.futures.as_completed(futures)]
  File "/usr/local/lib/python3.8/concurrent/futures/_base.py", line 437, in result
    return self.__get_result()
  File "/usr/local/lib/python3.8/concurrent/futures/_base.py", line 389, in __get_result
    raise self._exception
  File "/usr/local/lib/python3.8/concurrent/futures/thread.py", line 57, in run
    result = self.fn(*self.args, **self.kwargs)
  File "/tmpfs/src/github/google-auth-library-python/.nox/docs/lib/python3.8/site-packages/sphinx/ext/intersphinx.py", line 225, in fetch_inventory_group
    logger.warning(__("failed to reach any of the inventories "
  File "/usr/local/lib/python3.8/logging/__init__.py", line 1812, in warning
    self.log(WARNING, msg, *args, **kwargs)
  File "/tmpfs/src/github/google-auth-library-python/.nox/docs/lib/python3.8/site-packages/sphinx/util/logging.py", line 123, in log
    super().log(level, msg, *args, **kwargs)
  File "/usr/local/lib/python3.8/logging/__init__.py", line 1844, in log
    self.logger.log(level, msg, *args, **kwargs)
  File "/usr/local/lib/python3.8/logging/__init__.py", line 1512, in log
    self._log(level, msg, args, **kwargs)
  File "/usr/local/lib/python3.8/logging/__init__.py", line 1589, in _log
    self.handle(record)
  File "/usr/local/lib/python3.8/logging/__init__.py", line 1599, in handle
    self.callHandlers(record)
  File "/usr/local/lib/python3.8/logging/__init__.py", line 1661, in callHandlers
    hdlr.handle(record)
  File "/usr/local/lib/python3.8/logging/__init__.py", line 950, in handle
    rv = self.filter(record)
  File "/usr/local/lib/python3.8/logging/__init__.py", line 811, in filter
    result = f.filter(record)
  File "/tmpfs/src/github/google-auth-library-python/.nox/docs/lib/python3.8/site-packages/sphinx/util/logging.py", line 430, in filter
    raise exc
sphinx.errors.SphinxWarning: failed to reach any of the inventories with the following issues:
intersphinx inventory 'https://requests-oauthlib.readthedocs.io/en/stable/objects.inv' not fetchable due to <class 'requests.exceptions.HTTPError'>: 404 Client Error: Not Found for url: https://requests-oauthlib.readthedocs.io/en/stable/objects.inv

I have no idea why they would yank the stable docs version. The only versions I can see today for the requests-oauthlib docs are latest and v1.3.0-docs. Given that 1.3.0 is the latest release on PyPI, and is two years old, I think we're better off using the corresponding version.