`functools.lru_cache` has been in the standard library since Python 3.2.
The project requires Python >=3.10, so the `except ImportError` branch
importing `backports.functools_lru_cache` can never execute.
Remove the try/except block and keep only the direct stdlib import.
Also remove `types-backports` from dev dependencies, `backports` from
`[tool.deptry.known_first_party]`, and `types-backports` from the
DEP002 ignore list in `pyproject.toml`.
