Python agent lib soname major bump

Make the python agent library follow the main library soname for
co-installability with previous versions. Also use the value provided by
the build system instead of manually synchronizing it.

Change-Id: Ieef9b9ca6dcfd08b7834e5faafef3935a6a1a232
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>