[3.9] bpo-40204: Allow pre-Sphinx 3 syntax in the doc (GH-21844) (GH-21901)

* bpo-40204: Allow pre-Sphinx 3 syntax in the doc (GH-21844)

Enable Sphinx 3.2 "c_allow_pre_v3" option and disable the
c_warn_on_allowed_pre_v3 option to make the documentation compatible
with Sphinx 2 and Sphinx 3.

(cherry picked from commit 423e77d6de497931585d1883805a9e3fa4096b0b)

* bpo-40204: Fix Sphinx sytanx in howto/instrumentation.rst (GH-21858)

Use generic '.. object::' to declare markers, rather than abusing
'..  c:function::' which fails on Sphinx 3.

(cherry picked from commit 43577c01a2ab49122db696e9eaec6cb31d11cc81)

* bpo-40204: Fix duplicates in the documentation (GH-21857)

Fix two Sphinx 3 issues:

Doc/c-api/buffer.rst:304: WARNING: Duplicate C declaration, also defined in 'c-api/buffer'.
Declaration is 'PyBUF_ND'.

Doc/c-api/unicode.rst:1603: WARNING: Duplicate C declaration, also defined in 'c-api/unicode'.
Declaration is 'PyObject* PyUnicode_Translate(PyObject *str, PyObject *table, const char *errors)'.

(cherry picked from commit 46d10b1237c67ff8347f533eda6a5468d098f7eb)

* bpo-40204: Add :noindex: in the documentation (GH-21859)

Add :noindex: to duplicated documentation to fix "duplicate object
description" errors.

For example, fix this Sphinx 3 issue:

Doc/library/configparser.rst:1146: WARNING: duplicate object
description of configparser.ConfigParser.optionxform, other instance
in library/configparser, use :noindex: for one of them

(cherry picked from commit d3ded080482beae578faa704b13534a62d066f9f)

* bpo-40204, doc: Fix syntax of C variables (GH-21846)

For example, fix the following Sphinx 3 errors:

Doc/c-api/buffer.rst:102: WARNING: Error in declarator or parameters
Invalid C declaration: Expected identifier in nested name. [error at 5]
  void \*obj
  -----^

Doc/c-api/arg.rst:130: WARNING: Unparseable C cross-reference: 'PyObject*'
Invalid C declaration: Expected end of definition. [error at 8]
  PyObject*
  --------^

The modified documentation is compatible with Sphinx 2 and Sphinx 3.

(cherry picked from commit 474652fe9346382dbf793f20b671eb74668bebde)

* bpo-40204: Fix reference to terms in the doc (GH-21865)

Sphinx 3 requires to refer to terms with the exact case.

For example, fix the Sphinx 3 warning:

Doc/library/pkgutil.rst:71: WARNING: term Loader not found in case
sensitive match.made a reference to loader instead.

(cherry picked from commit bb0b08540cc93e56f3f1bde1b39ce086d9e35fe1)

* bpo-40204: Fix duplicated productionlist names in the doc (GH-21900)

Sphinx 3 disallows having more than one productionlist markup with
the same name. Simply remove names in this case, since names are not
shown anyway. For example, fix the Sphinx 3 warning:

Doc/reference/introduction.rst:96: duplicate token description
of *:name, other instance in reference/expressions

(cherry picked from commit 1abeda80f760134b4233608e2c288790f955b95a)
diff --git a/Doc/c-api/intro.rst b/Doc/c-api/intro.rst
index e89a788..7ca8693 100644
--- a/Doc/c-api/intro.rst
+++ b/Doc/c-api/intro.rst
@@ -229,13 +229,13 @@
 .. index:: object: type
 
 Most Python/C API functions have one or more arguments as well as a return value
-of type :c:type:`PyObject\*`.  This type is a pointer to an opaque data type
+of type :c:type:`PyObject*`.  This type is a pointer to an opaque data type
 representing an arbitrary Python object.  Since all Python object types are
 treated the same way by the Python language in most situations (e.g.,
 assignments, scope rules, and argument passing), it is only fitting that they
 should be represented by a single C type.  Almost all Python objects live on the
 heap: you never declare an automatic or static variable of type
-:c:type:`PyObject`, only pointer variables of type :c:type:`PyObject\*` can  be
+:c:type:`PyObject`, only pointer variables of type :c:type:`PyObject*` can  be
 declared.  The sole exception are the type objects; since these must never be
 deallocated, they are typically static :c:type:`PyTypeObject` objects.
 
@@ -496,7 +496,7 @@
 
 There are few other data types that play a significant role in  the Python/C
 API; most are simple C types such as :c:type:`int`,  :c:type:`long`,
-:c:type:`double` and :c:type:`char\*`.  A few structure types  are used to
+:c:type:`double` and :c:type:`char*`.  A few structure types  are used to
 describe static tables used to list the functions exported  by a module or the
 data attributes of a new object type, and another is used to describe the value
 of a complex number.  These will  be discussed together with the functions that