scripts: kdoc_parser.py: warn about Python version only once

scripts: kdoc_parser.py: warn about Python version only once

When running kernel-doc over multiple documents, it emits
one error message per file with is not what we want:

	$ python3.6 scripts/kernel-doc.py . --none
	...
	Warning: ./include/trace/events/swiotlb.h:0 Python 3.7 or later is required for correct results
	Warning: ./include/trace/events/iommu.h:0 Python 3.7 or later is required for correct results
	Warning: ./include/trace/events/sock.h:0 Python 3.7 or later is required for correct results
	...

Change the logic to warn it only once at the library:

	$ python3.6 scripts/kernel-doc.py . --none
	Warning: Python 3.7 or later is required for correct results
	Warning: ./include/cxl/features.h:0 Python 3.7 or later is required for correct results

When running from command line, it warns twice, but that sounds
ok.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Message-ID: <68e54cf8b1201d1f683aad9bc710a99421910356.1758196090.git.mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>