Python: examples: import explicit BT modules

Python: examples: import explicit BT modules

The best practice in Python is generally to import
explicit modules rather than importing all its exposed
members.

This also makes the examples hide the fact that
reader/writer members are exposed directly by the package,
whereas this is only a backward compatibility layer now.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>