nmt: use bytes instead of list for send_message and send_periodic

[bizfsc]

This PR extracts only the nmt.py changes from #651, which covers a broader set of mypy fixes across multiple files. Once the changes here are merged, #651 can be closed, as everything but the test_od changes are covered by other PRs.

The python-can send_message and send_periodic APIs expect bytes, not list[int]. Passing a list is accepted at runtime by some backends but is incorrect according to the type signatures and triggers Pylance and mypy errors. Using bytes literals and bytes() constructor makes the intent explicit and removes the type errors without any behavioral change.