Replaced pkg_resources with importlib.metadata

[idahogray]

Use of pkg_resources is discouraged in favor of importlib.resources
and importlib.metadata. importlib.metadata is used in this commit
to get the version string from the pyproject.toml package configuration.

This PR closes #22 in order to hopefully allow PyOxidizer to package the library.

[stefan6419846]

This will break compatibility in

checksumdir/pyproject.toml

Line 14 in 5c7a703

python = "^3.6"

as importlib.metadata is only available since Python 3.8. Older Python versions (like supported by this project) require using the backport in importlib-metadata.

[stefan6419846]

As already mentioned, just requiring importlib_metadata; python_version < "3.8" (or dropping Python < 3.8 completely) and writing

try:
    from importlib.metadata import version
except ImportError:
    # Python < 3.8
    from importlib_metadata import version

would fix this for every supported version since Python 3.6.

[amirmasoudabdol]

This now issue a warning as follow, and if not addressed it can very well break the package in a few months.

site-packages/checksumdir/__init__.py:17: 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.
  import pkg_resources

[amirmasoudabdol]

@to-mc, is there any way this being merged? 🙏🏼