converters, dumb: Fix errors since latest dulwich release (0.20.50)
ShaFile.get_type was deprecated and has been removed.
New typings have been added in dulwich that trigerred a new mypy error.
See https://jenkins.softwareheritage.org/job/DENV/job/tests/3220/artifact/swh-loader-git.log
Migrated from D8794 (view on Phabricator)