Check sorting of CONTRIB.rst vis pre-commit

[peterjc]

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

• 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.

• 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 99cbeb5

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

[peterjc]

Meant this to be one commit; the first attempt fails.

[peterjc]

Rebasing (wanted to double check this still works)