PEP 387: Update language around discussion venues (#4769)

Point to PEP 1, updated in #2266, for the venue list.

Author: cmaloney

peps/pep-0387.rst

@@ -146,9 +146,9 @@ Making Incompatible Changes
 Making an incompatible change is a gradual process performed over
 several releases:
 
-1. Discuss the change.  Depending on the degree of incompatibility,
-   this could be on the bug tracker, python-dev, python-list, or the
-   appropriate SIG.  A PEP or similar document may be written.
+1. :pep:`PEP 1: Start with an idea <1#start-with-an-idea-for-python>` discussion.
+   If the discussion reaches consensus that acceptance is possible a PEP or
+   similar document may be written.
    Hopefully users of the affected API will pipe up to comment.
 
 2. Add a warning to the current ``main`` branch.