python: define _BaseObject._ptr property

python: define _BaseObject._ptr property

Define the property `_ptr` on _BaseObject, which returns the value of
the currently-named `_ptr` field.  Rename the `_ptr` field to
`_ptr_internal`, for the lack of a better name.

The goal is for mixin classes to be able to declare that they require
such a property to exist, using `abc.abstractmethod`, and for
_BaseObject to fill in the implementation (and for static type checkers
to be happy).  For instance:

    class MyMixin(abc.ABC):
        @property
	@abc.abstractmethod
	def _ptr(self):
	    raise NotImplementedError

	def my_method(self):
	    ... do something with self._ptr ...

   class MyObject(object._SharedObject, MyMixin):
      ...

In this case, object._SharedObject provides the `_ptr` implementation
that `MyMixin` requires.

Change-Id: I696c04ed690ede7cd50bcb71890cc0b7b25f44e4
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/10326
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>