bt2: move __version__ to version.py.in

The only bit of __init__.py that is set at configure time is the package
version.  Move it to its own file, version.py.in, and make __init__.py
not generated.  This will make life a bit easier when we need to modify
__init__.py.

Change-Id: Ibc9fb67a897b226c46b4fddb3af7c2219d1f94e9
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1759
CI-Build: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>