update sphinx and fix sphinx warnings

link to the intersphinx inventory

https://alnoki.readthedocs.io/en/0.1.0/procedures/sphinx.html#using-intersphinx

.readthedocs.yml

@@ -1 +1,12 @@
 version: 2
+
+sphinx:
+  fail_on_warning: true
+
+python:
+  version: 3.8
+  install:
+    - method: pip
+      path: .
+      extra_requirements:
+        - docs

docs/conf.py

@@ -98,6 +98,7 @@
     "python": ("https://docs.python.org/3", None),
     "python2": ("https://docs.python.org/2", None),
     "six": ("https://six.readthedocs.io/", None),
+    "fissix": ("https://fissix.readthedocs.io/en/latest/", None),
 }
 
 
@@ -134,7 +135,7 @@
 # Add any paths that contain custom static files (such as style sheets) here,
 # relative to this directory. They are copied after the builtin static files,
 # so a file named "default.css" will overwrite the builtin "default.css".
-html_static_path = ["_static"]
+html_static_path = []
 
 # Add any extra paths that contain custom files (such as robots.txt or
 # .htaccess) here, relative to this directory. These files are copied

docs/fixers.rst

@@ -230,7 +230,7 @@ version of ``six`` is installed.
 
 
 ``fissix`` fixers
-+++++++++++++++
++++++++++++++++++
 
 Some `fixers from fissix <https://docs.python.org/3/library/2to3.html#fixers>`_
 in Python's standard library are run by default unmodified as their

setup.cfg

@@ -30,6 +30,10 @@ console_scripts =
     modernize = libmodernize.main:main
     python-modernize = libmodernize.main:main
 
+[options.extras_require]
+docs =
+  sphinx~=3.2
+
 [bdist_wheel]
 universal = 1