python.bs_slot: don't fail with runtime error if Python slot is deleted
python.bs_slot: don't fail with runtime error if Python slot is deleted

Python object derived from bs_slot can be destroyed while reference to
C++ bs_slot instance can still be held by some signal. If that signal
will be fired, bs_slot would fail with RuntimeError that pure virtual
method is called.

Fix that - if Python overload of execute() wasn't found just do nothing.

uentity/blue-sky-re,uentity/blue-sky-re,uentity/blue-sky-re