bpo-35042: Use the :pep: role where a PEP is specified (#10036)

diff --git a/Doc/library/test.rst b/Doc/library/test.rst
index 95d7f54..6a10bab 100644
--- a/Doc/library/test.rst
+++ b/Doc/library/test.rst
@@ -392,7 +392,7 @@
 
 .. function:: make_legacy_pyc(source)
 
-   Move a PEP 3147/488 pyc file to its legacy pyc location and return the file
+   Move a :pep:`3147`/:pep:`488` pyc file to its legacy pyc location and return the file
    system path to the legacy pyc file.  The *source* value is the file system
    path to the source file.  It does not need to exist, however the PEP
    3147/488 pyc file must exist.