Check sorting of CONTRIB.rst vis pre-commit

[peterjc]

• [x] I hereby agree to dual licence this and any previous contributions under both the Biopython License Agreement AND the BSD 3-Clause License.

• [x] I have read the CONTRIBUTING.rst file, have run pre-commit locally, and understand that continuous integration checks will be used to confirm the Biopython unit tests and style checks pass with these changes.

• [x] I have added my name to the alphabetical contributors listings in the files NEWS.rst and CONTRIB.rst as part of this pull request, am listed already, or do not wish to be listed. (This acknowledgement is optional.)

We used to run grep "^- " CONTRIB.rst | LC_ALL=C sort -u -c -f as part of the TravisCI setup (as documented at the start of that file), but appear not to be under GitHub Actions or AppVeyor.

Cross reference 99cbeb58a0d07ffc5a2fc55f11ae0dfad208fb56

This solution via pre-commit is not that elegant, perhaps there is an easier way - e.g. an entry in the GitHub actions directly?