python3-sphinx: remove BSD-3-Clause from LICENSE

License-Update: BSD-3-Clause code removed in
https://github.com/sphinx-doc/sphinx/commit/a7f5d91c29d6f377b9fe7e926965c6f9d3e7b802


which was a part of 7.0.1 update

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>