pmb.flasher.init: Assert that depends is a list (MR 2464)

pmaports_cfg.get() can return other types, too. As such, we need to
assert that it's a list to appease mypy.

pmb/flasher/init.py

@@ -36,6 +36,9 @@ def install_depends(method: str) -> None:
             ","
         )
 
+    if not isinstance(depends, list):
+        raise RuntimeError(f"depends was {type(depends)}, not a list")
+
     pmb.chroot.apk.install(depends, Chroot.native())