treewide: adopt pathlib.Path and type hinting (MR 2252)

With the new chroot type, we can now write fancy paths in the pythonic
way. Convert most of the codebase over, as well as adding various other
type hints.

Signed-off-by: Caleb Connolly <caleb@postmarketos.org>

pmb/flasher/variables.py

@@ -1,5 +1,6 @@
 # Copyright 2023 Oliver Smith
 # SPDX-License-Identifier: GPL-3.0-or-later
+from typing import Optional
 import pmb.config.pmaports
 from pmb.core.types import PmbArgs
 
@@ -15,6 +16,9 @@ def variables(args: PmbArgs, flavor, method):
     # updated and minimum pmbootstrap version bumped.
     # See also https://gitlab.com/postmarketOS/pmbootstrap/-/issues/2243
 
+    _partition_kernel: Optional[str]
+    _partition_rootfs: Optional[str]
+
     if method.startswith("fastboot"):
         _partition_kernel = args.deviceinfo["flash_fastboot_partition_kernel"]\
             or "boot"