remove use of pkg_resources

[braingram]

pkg_resources is deprecated. It's use in stdatamodels is limited to parsing the asdf version to check if it's at least 2.6. The pyproject.toml dependencies set a lower pin on asdf version 2.15.0 making the check in the code unnecessary: https://github.com/spacetelescope/stdatamodels/blob/27c397415ad2d96a1149f492b7906c49862fc7c2/pyproject.toml#L15

This PR removes the use of pkg_resources.

This is causing issues for 3.12 testing as seen in: https://github.com/spacetelescope/stdatamodels/pull/216

Regression tests passed with no errors: https://plwishmaster.stsci.edu:8081/job/RT/job/JWST-Developers-Pull-Requests/967/

Checklist

• [ ] added entry in CHANGES.rst (either in Bug Fixes or Changes to API)
• [ ] updated relevant tests
• [ ] updated relevant documentation
• [ ] updated relevant milestone(s)
• [ ] added relevant label(s)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (f3aa87e) 64.67% compared to head (db314e9) 64.66%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #217 +/- ## ========================================== - Coverage 64.67% 64.66% -0.01% ========================================== Files 102 102 Lines 5664 5660 -4 ========================================== - Hits 3663 3660 -3 + Misses 2001 2000 -1 ```

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[braingram]

CI Failures are unrelated and due to a mismatch introduced by an update to files on crds. If the mismatch was intentional it can be addressed with: https://github.com/spacetelescope/stdatamodels/pull/219