Fix FilePath creation on Python 3.12

[Mirochill]

Fixes #249.

Summary

• replace the private Path._from_parts() call in FilePath.__new__
• delegate FilePath construction to Path.__new__, which works with pathlib internals on Python 3.12+
• keep the existing FilePath dispatch to WindowsFilePath / PosixFilePath

Validation

• Not run locally.
• git diff --check
• git show --check HEAD
• rg -n "_from_parts|_flavour|Path\.__new__|class FilePath" telegram_upload\caption_formatter.py tests\test_caption_formatter.py