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

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• bpo-40204: Allow pre-Sphinx 3 syntax in the doc (bpo-40204: Allow pre-Sphinx 3 syntax in the doc #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 423e77d)

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

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

(cherry picked from commit 43577c0)

• bpo-40204: Fix duplicates in the documentation (bpo-40204: Fix duplicates in the documentation #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 46d10b1)

• bpo-40204: Add :noindex: in the documentation (bpo-40204: Add :noindex: in the documentation #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 d3ded08)

• bpo-40204, doc: Fix syntax of C variables (bpo-40204, doc: Fix syntax of C variables #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 474652f)

• bpo-40204: Fix reference to terms in the doc (bpo-40204: Fix reference to terms in the doc #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 bb0b085)

• bpo-40204: Fix duplicated productionlist names in the doc (bpo-40204: Fix duplicated productionlist names in the doc #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 1abeda8)
(cherry picked from commit 8f88190)

https://bugs.python.org/issue40204

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I got issues with Travis CI on my previous PR #21924 so I created this new PR to try to workaround the issue.