add if __main__ to changelog filter

[braingram]

The changelog_filter.py script added in #329 is causing issues with test collection. This can be seen in downstream tests in asdf
https://github.com/asdf-format/asdf/actions/runs/8065305926/job/22030878789?pr=1745

and by attempting to run pytest in a development install of dkist:

>> pytest
...
___________________________________________________ ERROR collecting .github/changelog_filter.py ____________________________________________________
.github/changelog_filter.py:10: in <module>
    source = input_stream.read()
E   io.UnsupportedOperation: not readable
...

This PR adds an if __name__ == '__main__' check to the script so it does not attempt to read stdin during test collection.

[braingram]

I wouldn't consider this in need of a changelog but let me know if adding one is preferred.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.38%. Comparing base (1043d38) to head (6fbcd7f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #338      +/-   ##
==========================================
- Coverage   97.73%   97.38%   -0.36%     
==========================================
  Files          34       34              
  Lines        1990     1990              
==========================================
- Hits         1945     1938       -7     
- Misses         45       52       +7     

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[Cadair]

Thanks!