Use Sphinx :pep: syntax to link PEPs (#5491)

Creating a news entry from the pull request as a separate commit.  Does this warrant an issue?

docs/reference/pip.rst

@@ -72,7 +72,7 @@ Build System Interface
 ======================
 
 Pip builds packages by invoking the build system. Presently, the only supported
-build system is ``setuptools``, but in the future, pip will support `PEP517`_
+build system is ``setuptools``, but in the future, pip will support :pep:`517`
 which allows projects to specify an alternative build system in a
 ``pyproject.toml`` file.  As well as package building, the build system is also
 invoked to install packages direct from source.  This is handled by invoking
@@ -118,7 +118,7 @@ PEP 518 Support
 
 As of 10.0, pip supports projects declaring dependencies that are required at
 install time using a ``pyproject.toml`` file, in the form described in
-`PEP518`_. When building a project, pip will install the required dependencies
+:pep:`518`. When building a project, pip will install the required dependencies
 locally, and make them available to the build process.
 
 When making build requirements available, pip does so in an *isolated
@@ -141,29 +141,27 @@ appropriately.
 
 **Limitations**:
 
-* until `PEP517`_ support is added, ``setuptools`` and ``wheel`` **must** be
+* until :pep:`517` support is added, ``setuptools`` and ``wheel`` **must** be
   included in the list of build requirements: pip will assume these as default,
   but will not automatically add them to the list of build requirements if
   explicitly defined in ``pyproject.toml``.
 
 * the current implementation only support installing build requirements from
   wheels: this is a technical limitation of the implementation - source
   installs would require a build step of their own, potentially recursively
-  triggering another `PEP518`_ dependency installation process. The possible
+  triggering another :pep:`518` dependency installation process. The possible
   unbounded recursion involved was not considered acceptable, and so
   installation of build dependencies from source has been disabled until a safe
   resolution of this issue is found.
 
 * ``pip<18.0`` does not support the use of environment markers and extras, only
   version specifiers are respected.
 
-.. _PEP517: https://www.python.org/dev/peps/pep-0517/
-.. _PEP518: https://www.python.org/dev/peps/pep-0518/
 
 Future Developments
 ~~~~~~~~~~~~~~~~~~~
 
-`PEP426`_ notes that the intention is to add hooks to project metadata in
+:pep:`426` notes that the intention is to add hooks to project metadata in
 version 2.1 of the metadata spec, to explicitly define how to build a project
 from its source. Once this version of the metadata spec is final, pip will
 migrate to using that interface. At that point, the ``setup.py`` interface
@@ -173,7 +171,6 @@ have migrated.
 Specifically, applications should *not* expect to rely on there being any form
 of backward compatibility guarantees around the ``setup.py`` interface.
 
-.. _PEP426: https://www.python.org/dev/peps/pep-0426/#metabuild-system
 
 Build Options
 ~~~~~~~~~~~~~

docs/reference/pip_install.rst

@@ -38,7 +38,7 @@ each is, in the following order:
    an error).
 3. Local file (a sdist or wheel format archive, following the naming
    conventions for those formats).
-4. A requirement, as specified in PEP 440.
+4. A requirement, as specified in :pep:`440`.
 
 Each item identified is added to the set of requirements to be satisfied by
 the install.
@@ -237,7 +237,7 @@ Requirement Specifiers
 pip supports installing from a package index using a :term:`requirement
 specifier <pypug:Requirement Specifier>`. Generally speaking, a requirement
 specifier is composed of a project name followed by optional :term:`version
-specifiers <pypug:Version Specifier>`.  `PEP508`_ contains a full specification
+specifiers <pypug:Version Specifier>`.  :pep:`508` contains a full specification
 of the format of a requirement (``pip`` does not support the ``url_req`` form
 of specifier at this time).
 
@@ -311,7 +311,7 @@ Pre-release Versions
 ++++++++++++++++++++
 
 Starting with v1.4, pip will only install stable versions as specified by
-`pre-releases`_ by default. If a version cannot be parsed as a compliant `PEP440`_
+`pre-releases`_ by default. If a version cannot be parsed as a compliant :pep:`440`
 version then it is assumed to be a pre-release.
 
 If a Requirement specifier includes a pre-release or development version
@@ -484,7 +484,7 @@ pip looks for packages in a number of places, on PyPI (if not disabled via
 ```--no-index```), in the local filesystem, and in any additional repositories
 specified via ```--find-links``` or ```--index-url```. There is no ordering in
 the locations that are searched, rather they are all checked, and the "best"
-match for the requirements (in terms of version number - see `PEP440`_ for
+match for the requirements (in terms of version number - see :pep:`440` for
 details) is selected.
 
 See the :ref:`pip install Examples<pip install Examples>`.
@@ -881,6 +881,3 @@ Examples
 
 .. [1] This is true with the exception that pip v7.0 and v7.0.1 required quotes
        around specifiers containing environment markers in requirement files.
-
-.. _PEP440: https://www.python.org/dev/peps/pep-0440/
-.. _PEP508: https://www.python.org/dev/peps/pep-0508/

docs/user_guide.rst

@@ -176,9 +176,7 @@ Installing from Wheels
 
 "Wheel" is a built, archive format that can greatly speed installation compared
 to building and installing from source archives. For more information, see the
-`Wheel docs <https://wheel.readthedocs.io>`_ ,
-`PEP427 <https://www.python.org/dev/peps/pep-0427/>`_, and
-`PEP425 <https://www.python.org/dev/peps/pep-0425/>`_
+`Wheel docs <https://wheel.readthedocs.io>`_ , :pep:`427`, and :pep:`425`.
 
 Pip prefers Wheels where they are available. To disable this, use the
 :ref:`--no-binary <install_--no-binary>` flag for :ref:`pip install`.