PEP 554: Fix citation references (#2644)

* PEP 554: Fix broken nathaniel-asyncio citation by converting to inline link

* PEP 554: Fix broken CSP citation

* PEP 554: Fix broken extension-docs citation by converting to inline link

* PEP 554: Convert orhpaned references to bullets

pep-0554.rst

@@ -519,7 +519,7 @@ Introducing an API for a new concurrency model, like happened with
 asyncio, is an extremely large project that requires a lot of careful
 consideration.  It is not something that can be done a simply as this
 PEP proposes and likely deserves significant time on PyPI to mature.
-(See `Nathaniel's post <nathaniel-asyncio>`_ on python-dev.)
+(See `Nathaniel's post <nathaniel-asyncio_>`_ on python-dev.)
 
 However, this PEP does not propose any new concurrency API.  At most
 it exposes minimal tools (e.g. subinterpreters, channels) which may
@@ -565,7 +565,7 @@ each with different goals.  Most center on correctness and usability.
 
 One class of concurrency models focuses on isolated threads of
 execution that interoperate through some message passing scheme.  A
-notable example is `Communicating Sequential Processes`_ (CSP) (upon
+notable example is Communicating Sequential Processes [CSP]_ (upon
 which Go's concurrency is roughly based).  The isolation inherent to
 subinterpreters makes them well-suited to this approach.
 
@@ -1014,7 +1014,7 @@ the following:
 
 A separate page will be added to the docs for resources to help
 extension maintainers ensure their modules can be used safely in
-subinterpreters, under `Extending Python <extension-docs>`_.  The page
+subinterpreters, under `Extending Python <extension-docs_>`_.  The page
 will include the following information:
 
 * a summary about subinterpreters (similar to the same in the new
@@ -1640,8 +1640,6 @@ References
 .. [c-api]
    https://docs.python.org/3/c-api/init.html#sub-interpreter-support
 
-.. _Communicating Sequential Processes:
-
 .. [CSP]
    https://en.wikipedia.org/wiki/Communicating_sequential_processes
    https://github.com/futurecore/python-csp
@@ -1674,19 +1672,12 @@ References
 .. [global-atexit]
    https://bugs.python.org/issue6531
 
-.. [mp-conn]
-   https://docs.python.org/3/library/multiprocessing.html#connection-objects
-
 .. [bug-rate]
    https://mail.python.org/pipermail/python-ideas/2017-September/047094.html
 
 .. [benefits]
    https://mail.python.org/pipermail/python-ideas/2017-September/047122.html
 
-.. [main-thread]
-   https://mail.python.org/pipermail/python-ideas/2017-September/047144.html
-   https://mail.python.org/pipermail/python-dev/2017-September/149566.html
-
 .. [reset_globals]
    https://mail.python.org/pipermail/python-dev/2017-September/149545.html
 
@@ -1706,12 +1697,18 @@ References
 .. [cache-line-ping-pong]
    https://mail.python.org/archives/list/python-dev@python.org/message/3HVRFWHDMWPNR367GXBILZ4JJAUQ2STZ/
 
-.. [nathaniel-asyncio]
+.. _nathaniel-asyncio:
    https://mail.python.org/archives/list/python-dev@python.org/message/TUEAZNZHVJGGLL4OFD32OW6JJDKM6FAS/
 
-.. [extension-docs]
+.. _extension-docs:
    https://docs.python.org/3/extending/index.html
 
+* mp-conn
+    https://docs.python.org/3/library/multiprocessing.html#connection-objects
+
+* main-thread
+    https://mail.python.org/pipermail/python-ideas/2017-September/047144.html
+    https://mail.python.org/pipermail/python-dev/2017-September/149566.html
 
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