Create py.typed (#189)

* Create py.typed

As recommended in [pep 0561](https://peps.python.org/pep-0561/), a blank `py.typed` marker should be included when type hints are used so downstream codes can type check with `mypy` and similar tools.

* Update py.typed

* fixed linting

* Update pyproject.toml

---------

Co-authored-by: BowenD-UCB <84425382+BowenD-UCB@users.noreply.github.com>

Author: Andrew-S-Rosen

chgnet/py.typed

@@ -43,7 +43,7 @@ Package = "https://pypi.org/project/chgnet"
 find = { include = ["chgnet*"], exclude = ["tests", "tests*"] }
 
 [tool.setuptools.package-data]
-"chgnet" = ["*.json"]
+"chgnet" = ["*.json", "py.typed"]
 "chgnet.pretrained" = ["*", "**/*"]
 
 [build-system]