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: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/doc/sphinx/cdomain.py b/doc/sphinx/cdomain.py
index cf13ff3..cbac8e6 100644
--- a/doc/sphinx/cdomain.py
+++ b/doc/sphinx/cdomain.py
@@ -48,7 +48,10 @@
 
 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__,