setup.cfg: make flake8 scan *.py.in files

setup.cfg: make flake8 scan *.py.in files

... in order to scan `src/bindings/python/bt2/setup.py.in` (it is a
valid Python program despite the name, the replacement tokens are only
in strings).

Make the pre-commit hooks run on this file as well (the technique is
described somewhere here [1]).

Doing so finds a small flake8 issue, fix it.

[1] https://pre-commit.com/#filtering-files-with-types

Change-Id: I1427d0d987884b72851741d6bb36620c5609ddb9
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/13492
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Michael Jeanson <mjeanson@efficios.com>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>