[ci skip] Modernize Sphinx css configuration

This adds a css file via the html_css_files option introduced in
Sphinx 1.8.  Our current use of html_context is broken in Sphinx
3.5.0.  It looks like they are going to fix that in
3.5.1 (sphinx-doc/sphinx#8893), but since the
modern way seems clearer, I switched to it.

docs/source/conf.py

@@ -73,11 +73,10 @@
 html_logo = 'mesa-logo-200.png'
 
 # Override theme stylesheet
-html_context = {
-    'css_files': [
-        '_static/theme_overrides.css',  # overrides for wide tables in RTD theme
-        ],
-    }
+html_css_files = [
+    'theme_overrides.css',  # overrides for wide tables in RTD theme
+]
+
 
 # standard substitutions
 rst_prolog = r"""