Solution was quite easy. Thanks to the user reply here: beehaw.org/comment/3535588 or programming.dev/comment/10034690 (Not sure if the links actually work as expected...)

Hi all. I have a little problem and don't know how to solve. A CLI program in Python is broken since Python 3.12. It was working in Python 3.11. The reason is, that Python 3.12 changed how subclassing of a pathlib.Path works (basically fixed an issue), which now breaks a workaround.

The class in question is:

class File(PosixPath):
    def __new__(cls, *args: Any, **kwargs: Any) -> Any:
        return cls._from_parts(args).expanduser().resolve()  # type: ignore

    def __init__(self, source: str | Path, *args: Any) -> None:
        super().__init__()
        self.__source = Path(source)

    @property
    def source(self) -> Path:
        return self.__source

    @property
    def modified(self) -> Time:
        return Time.fromtimestamp(os.path.getmtime(self))

    @property
    def changed(self) -> Time:
        return Time.fromtimestamp(os.path.getctime(self))

    @property
    def accessed(self) -> Time:
        return Time.fromtimestamp(os.path.getatime(self))

    # Calculate sha512 hash of self file and compare result to the
    # checksum found in given file. Return True if identical.
    def verify_sha512(self, file: File, buffer_size: int = 4096) -> bool:
        compare_hash: str = file.read_text().split(" ")[0]
        self_hash: str = ""
        self_checksum = hashlib.sha512()
        with open(self.as_posix(), "rb") as f:
            for chunk in iter(lambda: f.read(buffer_size), b""):
                self_checksum.update(chunk)
            self_hash = self_checksum.hexdigest()
        return self_hash == compare_hash

and I get this error when running the script:

Traceback (most recent call last):
File "/home/tuncay/.local/bin/geprotondl", line 1415, in 
sys.exit(main())
^^^^^^
File "/home/tuncay/.local/bin/geprotondl", line 1334, in main
arguments, status = parse_arguments(argv)
^^^^^^^^^^^^^^^^^^^^^
File "/home/tuncay/.local/bin/geprotondl", line 1131, in parse_arguments
default, status = default_install_dir()
^^^^^^^^^^^^^^^^^^^^^
File "/home/tuncay/.local/bin/geprotondl", line 1101, in default_install_dir
steam_root: File = File(path)
^^^^^^^^^^
File "/home/tuncay/.local/bin/geprotondl", line 97, in __new__
return cls._from_parts(args).expanduser().resolve()  # type: ignore
^^^^^^^^^^^^^^^
AttributeError: type object 'File' has no attribute '_from_parts'. Did you mean: '_load_parts'?

Now replacing _from_parts with _load_parts does not work either and I get this message in that case:

Traceback (most recent call last):
File "/home/tuncay/.local/bin/geprotondl", line 1415, in 
sys.exit(main())
^^^^^^
File "/home/tuncay/.local/bin/geprotondl", line 1334, in main
arguments, status = parse_arguments(argv)
^^^^^^^^^^^^^^^^^^^^^
File "/home/tuncay/.local/bin/geprotondl", line 1131, in parse_arguments
default, status = default_install_dir()
^^^^^^^^^^^^^^^^^^^^^
File "/home/tuncay/.local/bin/geprotondl", line 1101, in default_install_dir
steam_root: File = File(path)
^^^^^^^^^^
File "/home/tuncay/.local/bin/geprotondl", line 97, in __new__
return cls._load_parts(args).expanduser().resolve()  # type: ignore
^^^^^^^^^^^^^^^^^^^^^
File "/usr/lib/python3.12/pathlib.py", line 408, in _load_parts
paths = self._raw_paths
^^^^^^^^^^^^^^^
AttributeError: 'tuple' object has no attribute '_raw_paths'

I have searched the web and don't understand how to fix this. Has anyone an idea what to do?