Update RST syntax for linking CPython docs and PEPs

Author: webknjaz

aiosmtpd/docs/intro.rst

@@ -11,8 +11,7 @@ Local Mail Transfer Protocol (LMTP).  It is derived from
 standard library module, and provides both a command line interface and an API
 for use in testing applications that send email. Since the `smtpd module was
 removed in Python 3.12
-<https://docs.python.org/3.12/whatsnew/3.12.html#locale>`, following `PEP 594
-<https://peps.python.org/pep-0594/#smtpd>`, aiosmtpd is the recommended
+<https://docs.python.org/3.12/whatsnew/3.12.html#locale>`__, following :pep:`594#smtpd`, aiosmtpd is the recommended
 replacement.
 
 Inspiration for this library comes from several other packages: