using importlib.metadata to replace use of pkg_resources

[notmadcode]

Reason:
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.

Solution:
Replace the use of pkg_resources with importlib-based.
Then use of importlib.metadata to acces the metadata of sigtool installed.

Note:

1. Some changes are auto-format by black
2. This is not necessary, but i hate the warning every time I use my favorite tool.