Deprecate/retire GFFUtils and move functionality into genomics-bcftbx

[pjbriggs]

GFFutils is now largely unsupported and seems to be rarely used; it also has a name clash with a separate project which periodically causes confusion for external bioinformatians (see for example comment within to issue #36).

Additionally the automated testing for the package (which used Travis CI) not longer functions and needs to be replaced by a GitHub workflow, and the package itself has problems on at least one more modern Python release (see issue #52).

Therefore it seems desirable to deprecate this package , move any useful functionality across to the genomics-bcftbx library (https://github.com/fls-bioinformatics-core/genomics), and retire the documentation on ReadTheDocs. The actual path for deprecating a GitHub repository is not clear at this stage, however the following may be of some use:

• https://github.com/todogroup/guides/blob/master/shutting-down-an-open-source-project.md
• https://docs.github.com/en/repositories/archiving-a-github-repository/archiving-repositories
• https://github.com/caciobanu/improved-magento-layered-navigation (example of a project that is marked as end-of-life