Add SPDX info for `tests/utils/python/typing/typing.py`

Putting the info in a separate `.license` file to avoid modifying the
original one (which comes from the CPython project).

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: If183d9662f239da27a00b7f8ba8887eaec489270
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11383
Reviewed-by: Michael Jeanson <mjeanson@efficios.com>