docs: kdoc: emit a warning for ancient versions of Python

docs: kdoc: emit a warning for ancient versions of Python

Versions of Python prior to 3.7 do not guarantee to remember the insertion
order of dicts; since kernel-doc depends on that guarantee, running with
such older versions could result in output with reordered sections.

Python 3.9 is the minimum for the kernel as a whole, so this should not be
a problem, but put in a warning just in case somebody tries to use
something older.

Suggested-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Reviewed-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>