Replace deprecated pkg_resources import with importlib

[dennisushi]

Describe the issue

sapien/__init__.py unconditionally imports pkg_resources, which triggers a deprecation warning from setuptools>=81:

python3.12/site-packages/sapien/__init__.py:18: UserWarning: pkg_resources is deprecated as an API.
See https://setuptools.pypa.io/en/latest/pkg_resources.html.
The pkg_resources package is slated for removal as early as 2025-11-30.
Refrain from using this package or pin to Setuptools<81.

This affects both the 3.0.2 release and the nightly branch.

To Reproduce

import sapien

Expected behavior

No warning.

Suggested fix

Replace import pkg_resources with importlib.metadata / importlib.resources (stdlib since Python 3.9).