bt2: make `_ArrayField` inherit `collections.abc.MutableSequence`

bt2: make `_ArrayField` inherit `collections.abc.MutableSequence`

An array field is conceptually a mutable sequence just like an array
value (`ArrayValue`).

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Ia3f27fd524286e924b5529030963df33e7553336
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1562
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>