PEP 520: Fix citation references (#2637)

Co-authored-by: Adam Turner <[email protected]>

pep-0520.txt

@@ -344,8 +344,8 @@ be minimal.  All conforming implementations are expected to set
 Implementation
 ==============
 
-The implementation is found in the tracker. [impl_]
-
+The implementation is found in the
+`tracker <https://github.com/python/cpython/issues/68442>`__.
 
 Alternatives
 ============
@@ -410,21 +410,17 @@ discovery is almost entirely an implementation detail.
 References
 ==========
 
-.. [impl] issue #24254
-   (https://bugs.python.org/issue24254)
-
-.. [nick_concern] Nick's concerns about mutability
-   (https://mail.python.org/pipermail/python-dev/2016-June/144883.html)
-
-.. [orig] original discussion
-   (https://mail.python.org/pipermail/python-ideas/2013-February/019690.html)
+* `Original discussion
+  <https://mail.python.org/pipermail/python-ideas/2013-February/019690.html>`__
 
-.. [followup1] follow-up 1
-   (https://mail.python.org/pipermail/python-dev/2013-June/127103.html)
+* `Follow-up 1
+  <https://mail.python.org/pipermail/python-dev/2013-June/127103.html>`__
 
-.. [followup2] follow-up 2
-   (https://mail.python.org/pipermail/python-dev/2015-May/140137.html)
+* `Follow-up 2
+  <https://mail.python.org/pipermail/python-dev/2015-May/140137.html>`__
 
+* `Nick Coghlan's concerns about mutability
+  <https://mail.python.org/pipermail/python-dev/2016-June/144883.html>`__
 
 Copyright
 ===========