Merge branch 'mauro' into docs-mw

Mauro says:

This series  increases the minimal requirements for Sphinx and Python, and
drop some backward-compatible code from Sphinx extension.

Looking at Sphinx release dates:

	Release 2.4.0 (released Feb 09, 2020)
	Release 2.4.4 (released Mar 05, 2020) (current minimal requirement)
	Release 3.4.0 (released Dec 20, 2020)
	Release 3.4.3 (released Jan 08, 2021)

	(https://www.sphinx-doc.org/en/master/changes/index.html)

And Python release dates, we have:

	Python	Release date
	3.5	2015-09-13    (current minimal requirement)
	3.6	2016-12-23
	3.7 	2018-06-27
	3.8 	2019-10-14
	3.9 	2020-10-05
	3.10	2021-10-04

	(according with https://en.wikipedia.org/w/index.php?title=History_of_Python)

The new minimal requirements are now compatible with the toolset available on Jan, 2021,
e.g.:
	- Sphinx 3.4.3;
	- Python 3.9

The new Sphinx minimal requirement allows dropping all backward-compatible code
we have at kernel-doc and at Sphinx extensions.

The new Python minimal requirement also matches the current required level for
almost  all scripts (*).

Those matches a 4-years old toolchain, which sounds a reasonable period
of time, as Python/Sphinx aren't required for the Kernel build.

(*) Except for a couple scripts inside tools that require python 3.10:

    $ vermin -v $(git ls-files '*.py')|grep 3.10
    !2, 3.10     tools/net/sunrpc/xdrgen/generators/__init__.py
    !2, 3.10     tools/net/sunrpc/xdrgen/generators/program.py
    !2, 3.10     tools/net/sunrpc/xdrgen/subcmds/source.py
    !2, 3.10     tools/net/sunrpc/xdrgen/xdr_ast.py
    !2, 3.10     tools/perf/scripts/python/mem-phys-addr.py
    !2, 3.10     tools/power/cpupower/bindings/python/test_raw_pylibcpupower.py

Such scripts aren't required for Kernel builds, so it should be OK to set minimal
python version to 3.9.