Fix: doc/bindings/python/Makefile.am: run the `sphinx` Python 3 package

Fix: doc/bindings/python/Makefile.am: run the `sphinx` Python 3 package

In `sphinx-build.py` (removed), importing `main` and `make_main` from
the `sphinx` package is not considered stable: it used to work for me,
but now it doesn't (cannot import). Instead, run the `sphinx` package
directly with the Python interpreter's `-m` option.

We do this instead of running `sphinx-build` directly because we want to
pick Sphinx for Python 3 (because we're importing the `bt2` package for
autodoc), not for Python 2, and the installed `sphinx-build` program can
be the one for Python 2 on some distributions.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I158dbb96a8b3a26db67dd6e16254a64ac9d24446
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1401