bt2: add local copy of typing module

bt2: add local copy of typing module

Add a local copy of the typing module [1], in order to be able to use
the typing module features with Python 3.4.

Add notes about the license in the top-level LICENSE file.

Add exclusions to avoid this file being reformatted by black and
analyzed by flake8.

Add an import in utils.py, such that other files will be able to use the
typing module using:

    typing = utils._typing_mod

... and not care where the typing module comes from.

[1] https://github.com/python/typing/blob/3.10.0.2/src/typing.py

Change-Id: Id36fb65804ad8e00962b70f185c99325e314fee7
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/10300
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>