doc: Make python bindings doc build

Babeltrace 1 Python bindings (the babeltrace package) were removed in
commit

    1c81bd8895c0 ("Remove babeltrace 1 Python bindings and tests")

However, the Python doc still references it.  It tries to import it to
get the version, and therefore fails to build.  Update it to import the
bt2 package instead.

Change-Id: I656a89ad83069f6794abf4c775a14fb9be2ecba0
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1124
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>