selftests: net: py: coding style improvements

selftests: net: py: coding style improvements

We're about to add more features here and finding new issues with old
ones in place is hard. Address ruff checks:
 - bare exceptions
 - f-string with no params
 - unused import

We need to use BaseException when handling defer(), as Petr points out.
This retains the old behavior of ignoring SIGTERM while running cleanups.

Reviewed-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Link: https://patch.msgid.link/20251120021024.2944527-2-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>