Newer versions of python (3.13+ https://docs.python.org/3/library/dbm.html#module-dbm.sqlite3)
use sqlite as a key/value store replacing the use of (g)dbm, bdb.

This will be a win on windows where dbm backend devolved to dumbdbm which performs
poorly.

However as msg8151 indicates, there may be issues with tests.

So far 3.13 runs seem to complete ok in CI:

   https://github.com/roundup-tracker/roundup/actions/runs/11394478475/job/31704876042

but I can't tell if it's using the sqlite backend.

This issue tracks any changes required to support the back end change and verify
that 3.13 tests are using and working ok with sqlite.

msg8499 docs fixes for read only handles that were not closed in the dbm
module.
The sqlite change ended up cause ResourceWarning errors.