Taught PulpImporter to retry (once) on _import_file() failure.
There's a race condition in django-import-export's get_or_init_instance() that is exercised by importing repo-versions concurrently. We attempt an import and check for errors, retrying ONCE if encountered. On a second error, fail the attempt.
The test added for pulp_rpm #7904 cover this case.
fixes #8633 [nocoverage]