Remove Pydantic and update remaining dependencies

Pydantic was used only for validating settings. Since we don't have any
really complex types there, we can easily do it manually. We would have
to upgrade to Pydantic V2 soon, and that means +6.5MB of dependencies,
so let's drop it and enjoy a smaller install venv.

itch_dl/cli.py

@@ -22,7 +22,7 @@ def parse_args() -> argparse.Namespace:
                         help="configuration profile to load")
 
     # These args must match config.py -> Settings class. Make sure all defaults here
-    # evaluate to False, or apply_args_on_settings will override profile settings.
+    # evaluate to False, or load_config will override profile settings.
     parser.add_argument("--api-key", metavar="key", default=None,
                         help="itch.io API key - https://itch.io/user/settings/api-keys")
     parser.add_argument("--user-agent", metavar="agent", default=None,