Fix sphinxext related issues

Sphinx 6.2.1 is the last version of Sphinx that supports Python 3.8, so
this updates the dev requirements to use that.

There were changes in the ast module, so this updates the sphinxext code
to stop using deprecated things.

This adds "sphinx" as an extra and adds that to the relevant
documentation.

README.rst

@@ -54,13 +54,17 @@ Run::
 
 Some configuration environments require additional dependencies::
 
-
     # For INI support
     $ pip install 'everett[ini]'
 
     # for YAML support
     $ pip install 'everett[yaml]'
 
+Additionally, if you want to use the Sphinx extension to document
+your configuration, you need to add Sphinx::
+
+    # to use the Sphinx extension
+    $ pip install 'everett[sphinx]'
 
 Quick start
 ===========

requirements-dev.txt

@@ -1,5 +1,5 @@
 # Requirements file for developing on Everett.
--e .[ini,yaml]
+-e .[ini,yaml,sphinx]
 
 black==23.9.1
 check-manifest==0.49
@@ -12,6 +12,6 @@ tox-gh-actions==3.1.3
 twine==4.0.2
 types-PyYAML==6.0.12.12
 
-# These require Python 3.8+ before we can update them
-Sphinx==5.3.0
-sphinx_rtd_theme==1.0.0
+# We need to drop support for Python 3.8 before we can update this further.
+Sphinx==6.2.1
+sphinx_rtd_theme==1.3.0

setup.py

@@ -24,6 +24,7 @@ def get_file(fn):
 
 INSTALL_REQUIRES = []
 EXTRAS_REQUIRE = {
+    "sphinx": ["sphinx"],
     "ini": ["configobj"],
     "yaml": ["PyYAML"],
 }

src/everett/sphinxext.py

@@ -2,7 +2,13 @@
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
-"""Sphinx extension for auto-documenting components with configuration."""
+"""Sphinx extension for auto-documenting components with configuration.
+
+To use this, you must install the optional requirements::
+
+    $ pip install 'everett[sphinx]'
+
+"""
 
 import ast
 from importlib import import_module
@@ -705,12 +711,12 @@ def extract_configuration(
                 # }
                 for key, val in zip(node.keys, node.values):
                     if (
-                        isinstance(key, ast.Str)
+                        isinstance(key, ast.Constant)
                         and isinstance(val, ast.Call)
                         and isinstance(val.func, ast.Name)
                         and val.func.id == variable_name
                     ):
-                        config_nodes.append((key.s, val))
+                        config_nodes.append((key.value, val))
 
         CONFIG_ARGS = [
             "key",