Set sphinx_rtd_theme as html_theme + extensions

...as per the installation instructions at
https://sphinx-rtd-theme.readthedocs.io/en/1.3.0/installing.html.
This should admittedly have been done in
c3119b0, when originally adding
`sphinx-rtd-theme` as a requirement.

Lastly, it seems like the `html_theme` config setting was recently made
required, since only today's builds fail (see e.g.
https://readthedocs.org/projects/django-simple-history/builds/21687449/).

docs/conf.py

@@ -25,7 +25,10 @@
 
 # Add any Sphinx extension module names here, as strings. They can be extensions
 # coming with Sphinx (named 'sphinx.ext.*') or your custom ones.
-extensions = ["sphinx.ext.autodoc"]
+extensions = [
+    "sphinx.ext.autodoc",
+    "sphinx_rtd_theme",
+]
 
 # Add any paths that contain templates here, relative to this directory.
 templates_path = ["_templates"]
@@ -93,7 +96,7 @@
 
 # The theme to use for HTML and HTML Help pages.  See the documentation for
 # a list of builtin themes.
-# html_theme = 'default'
+html_theme = "sphinx_rtd_theme"
 
 # Theme options are theme-specific and customize the look and feel of a theme
 # further.  For a list of options available for each theme, see the