Avoid using concurrency with Sphinx 3.0 due to a bug in that version

sphinx-doc/sphinx#7438
#2324

src/scripts/build_docs.py

@@ -139,7 +139,14 @@ def sphinx_supports_concurrency():
     match = re.match(r'^(?:[a-zA-Z_-]+) v?(([0-9]+)\.([0-9]+))', output)
     # default to using concurrency when uncertain
     version = StrictVersion(match.group(1)) if match else StrictVersion('1.2')
-    return version >= StrictVersion('1.4')
+
+    if version < StrictVersion('1.4'):
+        # not supported
+        return False
+    if version >= StrictVersion('3.0'):
+        # Bug in Sphinx 3.0 https://github.com/sphinx-doc/sphinx/issues/7438
+        return False
+    return True
 
 def read_config(config):
     try: