tools/docs/get_feat.py: convert get_feat.pl to Python

tools/docs/get_feat.py: convert get_feat.pl to Python

As we want to call Python code directly at the Sphinx extension,
convert get_feat.pl to Python.

The code was made to be (almost) bug-compatible with the Perl
version, with two exceptions:

1. Currently, Perl script outputs a wrong table if arch is set
   to a non-existing value;

2. the ReST table output when --feat is used without --arch
   has an invalid format, as the number of characters for the
   table delimiters are wrong.

Those two bugs were fixed while testing the conversion.

Additionally, another caveat was solved:
the output when --feat is used without arch and the feature
doesn't exist doesn't contain an empty table anymore.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <03c26cee1ec567804735a33047e625ef5ab7bfa8.1763492868.git.mchehab+huawei@kernel.org>