checkpatch: Set explicit encoding in do_authors_exist.

checkpatch: Set explicit encoding in do_authors_exist.

The UTF-8 encoding system is used by default on most operating systems, but
not all of them. This can cause the ’24:checkpatch - AUTHORS.rst existence’
test to fail with a UnicodeDecodeError. All Python versions < 3.15 are
affected, resolved in PEP 686 (https://peps.python.org/pep-0686/)

Fixes: a6ccd111552d ("checkpatch: Add new check-authors-file option to checkpatch.py.")
Acked-by: Aaron Conole <aconole@redhat.com>
Signed-off-by: Ivan Burnin <iburnin@k2.cloud>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>