Mark PEP mentions as :pep: SphinX refs

docs/setuptools.txt

@@ -2200,22 +2200,23 @@ setup.cfg-only projects
 
 .. versionadded:: 40.9.0
 
-If ``setup.py`` is missing from the project directory when a PEP 517
+If ``setup.py`` is missing from the project directory when a :pep:`517`
 build is invoked, ``setuptools`` emulates a dummy ``setup.py`` file containing
 only a ``setuptools.setup()`` call.
 
 .. note::
 
-    PEP 517 doesn't support editable installs so this is currently
+    :pep:`517` doesn't support editable installs so this is currently
     incompatible with ``pip install -e .``.
 
 This means that you can have a Python project with all build configuration
 specified in ``setup.cfg``, without a ``setup.py`` file, if you **can rely
-on** your project always being built by a PEP 517/518 compatible frontend.
+on** your project always being built by a :pep:`517`/:pep:`518` compatible
+frontend.
 
 To use this feature:
 
-* Specify build requirements and PEP 517 build backend in
+* Specify build requirements and :pep:`517` build backend in
   ``pyproject.toml``.
   For example:
 
@@ -2228,11 +2229,11 @@ To use this feature:
       ]
       build-backend = "setuptools.build_meta"
 
-* Use a PEP 517 compatible build frontend, such as ``pip >= 19`` or ``pep517``.
+* Use a :pep:`517` compatible build frontend, such as ``pip >= 19`` or ``pep517``.
 
   .. warning::
 
-      As PEP 517 is new, support is not universal, and frontends that
+      As :pep:`517` is new, support is not universal, and frontends that
       do support it may still have bugs. For compatibility, you may want to
       put a ``setup.py`` file containing only a ``setuptools.setup()``
       invocation.