remove use of deprecated override__dir__

[braingram]

astropy 5.2 will deprecate override__dir__
The deprecation notes suggest an alternative (used here) which should work for all versions of python >=3.3

Deprecation: https://github.com/astropy/astropy/blob/0944607116f373e64e94ce3e5535c49509897c87/astropy/utils/compat/misc.py#L19
Root python issue: https://bugs.python.org/issue12166
Fixed in 3.3: https://docs.python.org/3.3/whatsnew/changelog.html

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]

Codecov Report

Base: 83.78% // Head: 83.78% // No change to project coverage 👍

Coverage data is based on head (11721d8) compared to base (11721d8).
Patch has no changes to coverable lines.

❗ Current head 11721d8 differs from pull request most recent head 638481d. Consider uploading reports for the commit 638481d to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #103   +/-   ##
=======================================
  Coverage   83.78%   83.78%           
=======================================
  Files          10       10           
  Lines        1714     1714           
=======================================
  Hits         1436     1436           
  Misses        278      278           

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report at Codecov.
📢 Do you have feedback about the report comment? Let us know in this issue.

[jdavies-st]

Perfect. This code is actually tested by the jwst.datamodels unit tests, not the stdatamodels tests, hence coverage not showing it. Would be good to add the jwst.datamodels tests to coverage.