force sphinx version for readthedocs

docs/conf.py

@@ -51,7 +51,6 @@
 #   '/_static/api/': [r'rdflib\.'],
 #   }
 
-
 # The suffix of source filenames.
 source_suffix = ".rst"
 
@@ -69,6 +68,7 @@
 # |version| and |release|, also used in various other places throughout the
 # built documents.
 
+
 # Find version. We have to do this because we can't import it in Python 3 until
 # its been automatically converted in the setup process.
 def find_version(filename):
@@ -238,3 +238,5 @@ def find_version(filename):
 }
 
 html_experimental_html5_writer = True
+
+needs_sphinx = "2.4"