Use platformdirs to find user configuration files

The previous library we were using for this (`appdirs`) is dead upstream
and not supported anymore in debian testing.

Fixes #1058

dangerzone/util.py

@@ -5,11 +5,14 @@ import sys
 import traceback
 import unicodedata
 
-import appdirs
+try:
+    import platformdirs
+except ImportError:
+    import appdirs as platformdirs
 
 
 def get_config_dir() -> str:
-    return appdirs.user_config_dir("dangerzone")
+    return platformdirs.user_config_dir("dangerzone")
 
 
 def get_resource_path(filename: str) -> str:

pyproject.toml

@@ -15,7 +15,7 @@ include = [
 [tool.poetry.dependencies]
 python = ">=3.9,<3.13"
 click = "*"
-appdirs = "*"
+platformdirs = "*"
 PySide6 = "^6.7.1"
 PyMuPDF = "^1.23.3"  # The version in Fedora 39
 colorama = "*"