Pin docutils < 0.18 to fix readthedocs rendering

RTD installs this project with `pip install -U --upgrade-strategy eager .`
which pulls in docutils via `readme_renderer`.  Pinning `docutils` even
though we don't directly depend on it clears up the rendering errors.

setup.cfg

@@ -37,6 +37,12 @@ python_requires = >=3.6
 install_requires=
     pkginfo >= 1.4.2
     readme_renderer >= 21.0
+    # XXX: 2Nov21: Current versions of Sphinx do not work with docutils 0.18,
+    #      which is pulled in by readme_renderer when readthedocs tries to
+    #      build the docs.  This pin should be removed as soon as Sphinx is
+    #      updated to use docutils 0.18 or readme_renderer is updated to pin
+    #      docutils<0.18, whichever comes first.
+    docutils < 0.18
     requests >= 2.20
     requests-toolbelt >= 0.8.0, != 0.9.0
     tqdm >= 4.14