Undo sphinx pin

CHANGES.rst

@@ -9,9 +9,6 @@ Changelog
 
 -   Fix a bug in configuration of the Jinja template directory.
 
--   Restrict documentation build to sphinx < 4 due to an API change
-    in that version that impacts Celery 4.x.
-
 0.13.1 (2021-03-01)
 -------------------
 

docs-requirements.txt

 celery[redis]>=5.1
 matplotlib
-sphinx>=2.0, <4  # FIXME: Sphinx 4 removed the symbol sphinx.domains.python.PyModulelevel, which is reference by Celery 4. Revisit this version constraint after we upgrade to Celery 5.
+sphinx>=2.0