Bump up to Sphinx 4

pradyunsg · pradyunsg · commit 086a85150d75 · 2021-07-28T10:49:25.000+01:00
Using the newer versions of Sphinx enables using newer versions of the
Sphinx theme we use.
diff --git a/docs/requirements.txt b/docs/requirements.txt
@@ -1,4 +1,4 @@
-sphinx == 3.2.1
+sphinx ~= 4.1.0
 towncrier
 furo
 myst_parser
