pmb.chroot.apk: Import annotations from __future__ (MR 2425)

Works around circular import.

pmb/chroot/apk.py

 # Copyright 2023 Oliver Smith
 # SPDX-License-Identifier: GPL-3.0-or-later
+
+from __future__ import annotations
+
 import os
 from pathlib import Path
 import traceback