Use :pep: role for soft deprecation link

Author: encukou

peps/pep-0820.rst

@@ -513,7 +513,7 @@ unique IDs:
 Soft deprecation
 ----------------
 
-These existing functions will be `soft-deprecated <https://peps.python.org/pep-0387/#soft-deprecation>`__:
+These existing functions will be :pep:`soft-deprecated <387#soft-deprecation>`:
 
 - ``PyType_FromSpec``
 - ``PyType_FromSpecWithBases``