pybind: switch from pkgutil.find_loader() to importlib.util.find_spec()

pybind: switch from pkgutil.find_loader() to importlib.util.find_spec()

Replace pkgutil.find_loader() with importlib.util.find_spec() throughout
Python bindings. This addresses the deprecation warning in Python 3.10
(scheduled for removal in 3.14) that appeared when generating librbd
Python bindings.

The importlib.util.find_spec() API has been available since Python 3.4
and is compatible with our minimum required Python version (3.9, since
commit 51f71fc1).

The warning resolved:
```
/home/kefu/dev/ceph/src/pybind/rbd/setup.py:8: DeprecationWarning: 'pkgutil.find_loader' is deprecated and slated for removal in Python 3.14; use importlib.util.find_spec() instead
  if not pkgutil.find_loader('setuptools'):
```

Signed-off-by: Kefu Chai <tchaikov@gmail.com>