Run ruff check --fix (MR 2357)

Now that we have target-version = "py310" in [tool.ruff] in
pyproject.toml, ruff check complains about using typing.Optional and
typing.Union instead of newer syntax. Run the tool to fix it.

pmb/flasher/variables.py

@@ -1,6 +1,5 @@
 # Copyright 2023 Oliver Smith
 # SPDX-License-Identifier: GPL-3.0-or-later
-from typing import Optional
 import pmb.config.pmaports
 from pmb.core.chroot import Chroot
 from pmb.core.context import get_context
@@ -20,8 +19,8 @@ def variables(args: PmbArgs, flavor: str, method: str):
     # updated and minimum pmbootstrap version bumped.
     # See also https://gitlab.com/postmarketOS/pmbootstrap/-/issues/2243
 
-    _partition_kernel: Optional[str]
-    _partition_rootfs: Optional[str]
+    _partition_kernel: str | None
+    _partition_rootfs: str | None
 
     if method.startswith("fastboot"):
         _partition_kernel = deviceinfo.flash_fastboot_partition_kernel or "boot"