Add PEP substitution in changelog.

docs/conf.py

@@ -42,6 +42,10 @@ link_files = {
 				pattern=r"^(?m)((?P<scm_version>v?\d+(\.\d+){1,2}))\n[-=]+\n",
 				with_scm="{text}\n{rev[timestamp]:%d %b %Y}\n",
 			),
+			dict(
+				pattern=r"PEP[- ](?P<pep_number>\d+)",
+				url='https://www.python.org/dev/peps/pep-{pep_number:0>4}/',
+			),
 		],
 	),
 }