PEP 440: note that setuptools comparison is historical (#2422)

Author: ncoghlan

pep-0440.txt

@@ -1204,6 +1204,10 @@ defined in a new PEP.
 Summary of differences from pkg_resources.parse_version
 =======================================================
 
+* Note: this comparison is to ``pkg_resourses.parse_version`` as it existed at
+  the time the PEP was written. After the PEP was accepted, setuptools 6.0 and
+  later versions adopted the behaviour described in this PEP.
+
 * Local versions sort differently, this PEP requires that they sort as greater
   than the same version without a local version, whereas
   ``pkg_resources.parse_version`` considers it a pre-release marker.