Add configuration for python code formatter black

Black will look for pyproject.toml automatically.
pyproject.toml is defined by PEP518 [1]

Use skip-string-normalization for now [2].

Exclude results from swig, build directory of the python bindings and
results of python input file (py.in).

Include py.in files.

[1] https://www.python.org/dev/peps/pep-0518/
[2] https://black.readthedocs.io/en/stable/the_black_code_style.html#strings

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Change-Id: I57adffcba9517dea05f10f9571671e75ac1a860f
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1355
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>