PEP 593: Fix citation references (#2640)

* PEP 593: Remove orphan citation to fix build warning

* PEP 593: Inline the references to fix warnings

pep-0593.rst

@@ -43,13 +43,14 @@ should ignore it and simply treat the type as ``T``. Unlike the
 ``no_type_check`` functionality that currently exists in the ``typing``
 module which completely disables typechecking annotations on a function
 or a class, the ``Annotated`` type allows for both static typechecking
-of ``T`` (e.g., via mypy [mypy]_ or Pyre [pyre]_, which can safely ignore ``x``)
+of ``T`` (e.g., via `mypy <mypy_>`_ or `Pyre <pyre_>`_,
+which can safely ignore ``x``)
 together with runtime access to ``x`` within a specific application. The
 introduction of this type would address a diverse set of use cases of interest
 to the broader Python community.
 
-This was originally brought up as issue 600 [issue-600]_ in the typing github
-and then discussed in Python ideas [python-ideas]_.
+This was originally brought up as `issue 600 <issue-600_>`_ in the typing github
+and then discussed in `Python ideas <python-ideas_>`_.
 
 Motivating examples
 -------------------
@@ -81,21 +82,21 @@ Lowering barriers to developing new typing constructs
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Typically when adding a new type, a developer need to upstream that type to the
-typing module and change mypy, PyCharm [pycharm]_, Pyre, pytype [pytype]_,
+typing module and change mypy, `PyCharm <pycharm_>`_, Pyre, `pytype <pytype_>`_,
 etc...
 This is particularly important when working on open-source code that
 makes use of these types, seeing as the code would not be immediately
 transportable to other developers' tools without additional logic. As a result,
 there is a high cost to developing and trying out new types in a codebase.
 Ideally, authors should be able to introduce new types in a manner that allows
-for graceful degradation (e.g.: when clients do not have a custom mypy plugin
-[mypy-plugin]_), which would lower the barrier to development and ensure some
+for graceful degradation (e.g.: when clients do not have a custom `mypy plugin
+<mypy-plugin_>`_), which would lower the barrier to development and ensure some
 degree of backward compatibility.
 
-For example, suppose that an author wanted to add support for tagged unions
-[tagged-union]_ to Python. One way to accomplish would be to annotate
-``TypedDict`` [typed-dict]_ in Python such that only one field is allowed to be
-set::
+For example, suppose that an author wanted to add support for `tagged unions
+<tagged-union_>`_ to Python. One way to accomplish would be to `annotate
+<typed-dict_>`_ ``TypedDict`` in Python such that only one field is allowed
+to be set::
 
     Currency = Annotated[
         TypedDict('Currency', {'dollars': float, 'pounds': float}, total=False),
@@ -236,7 +237,7 @@ cause ``Annotated`` to not integrate cleanly with the other typing annotations:
 * ``Annotated`` cannot infer the decorated type. You could imagine that
   ``Annotated[..., Immutable]`` could be used to mark a value as immutable
   while still inferring its type. Typing does not support using the
-  inferred type anywhere else [issue-276]_; it's best to not add this as a
+  inferred type `anywhere else <issue-276_>`_; it's best to not add this as a
   special case.
 
 * Using ``(Type, Ann1, Ann2, ...)`` instead of
@@ -253,40 +254,34 @@ This feature was left out to keep the design simple:
   little benefit.
 
 
-References
-----------
-
-.. [issue-600]
+.. _issue-600:
    https://github.com/python/typing/issues/600
 
-.. [python-ideas]
+.. _python-ideas:
    https://mail.python.org/pipermail/python-ideas/2019-January/054908.html
 
-.. [struct-doc]
-   https://docs.python.org/3/library/struct.html#examples
-
-.. [mypy]
+.. _mypy:
    http://www.mypy-lang.org/
 
-.. [pyre]
+.. _pyre:
    https://pyre-check.org/
 
-.. [pycharm]
+.. _pycharm:
    https://www.jetbrains.com/pycharm/
 
-.. [pytype]
+.. _pytype:
    https://github.com/google/pytype
 
-.. [mypy-plugin]
+.. _mypy-plugin:
    https://github.com/python/mypy_extensions
 
-.. [tagged-union]
+.. _tagged-union:
    https://en.wikipedia.org/wiki/Tagged_union
 
-.. [typed-dict]
+.. _typed-dict:
    https://mypy.readthedocs.io/en/latest/more_types.html#typeddict
 
-.. [issue-276]
+.. _issue-276:
    https://github.com/python/typing/issues/276
 
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