scripts: kdoc: make it backward-compatible with Python 3.7

scripts: kdoc: make it backward-compatible with Python 3.7

There was a change at kdoc that ended breaking compatibility
with Python 3.7: str.removesuffix() was introduced on version
3.9.

Restore backward compatibility.

Reported-by: Akira Yokosawa <akiyks@gmail.com>
Closes: https://lore.kernel.org/linux-doc/57be9f77-9a94-4cde-aacb-184cae111506@gmail.com/
Fixes: 27ad33b6b349 ("kernel-doc: Fix symbol matching for dropped suffixes")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/d13058d285838ac2bc04c492e60531c013a8a919.1752218291.git.mchehab+huawei@kernel.org