PEP 554: Fix citation references

[hugovk]

pep-0554.rst:1709: WARNING: Citation [nathaniel-asyncio] is not referenced.
pep-0554.rst:1712: WARNING: Citation [extension-docs] is not referenced.

Fixed by converting to inline links.

---

pep-0554.rst:1645: WARNING: Citation [CSP] is not referenced.

This citation was added in #419.

The text uses:

`Communicating Sequential Processes`_ (CSP)

and:

.. _Communicating Sequential Processes:

.. [CSP]
   https://en.wikipedia.org/wiki/Communicating_sequential_processes
   https://github.com/futurecore/python-csp

So let's change this to a single citation:

Communicating Sequential Processes [CSP]_

and:

.. [CSP]
   https://en.wikipedia.org/wiki/Communicating_sequential_processes
   https://github.com/futurecore/python-csp

---

pep-0554.rst:1677: WARNING: Citation [mp-conn] is not referenced.

This citation was added in #424, but removed/orphaned in #1390.

Let's remove the orphan.

---

pep-0554.rst:1686: WARNING: Citation [main-thread] is not referenced.

Similarly, added in #426 and removed/orphaned in #944.

[ericsnowcurrently]

Please leave orphaned references. The linked information still captures relevant information. It's fine if they are turned into bullets, or something similar, in the References section.

Other than that, LGTM.

[hugovk]

Updated! How does this look?

https://pep-previews--2644.org.readthedocs.build/pep-0554/#references

[ericsnowcurrently]

LGTM

Thanks for the improvements!

[hugovk]

Thanks for the reviews!