Use future type annotations

[janosh]

Using the side-effect from __future__ import annotations, Python backports support for the 3.10 type annotation syntax all the way to 3.7.

[janosh]

This line isort.required-imports = ["from __future__ import annotations"] in pyproject.toml makes ruff auto-add this import at the start of every file. This line and all the imports it adds can be removed once the minimum supported Python version for chgnet is 3.10.