doc: Sphinx.override_domain() deprecated

Sphinx.override_domain() is deprecated since Sphinx 1.8 and removed in
Sphinx 3.

Use Sphinx.add_domain(, override=True) instead.
Signed-off-by: NHeinrich Schuchardt <xypron.glpk@gmx.de>

doc/sphinx/cdomain.py

@@ -48,7 +48,10 @@ major, minor, patch = sphinx.version_info[:3]
 
 def setup(app):
 
-    app.override_domain(CDomain)
+    if (major == 1 and minor < 8):
+        app.override_domain(CDomain)
+    else:
+        app.add_domain(CDomain, override=True)
 
     return dict(
         version = __version__,