dist: add bzip3 support. From https://bugs.gnu.org/73795 (automake-patches). * bin/automake.in (handle_dist): add bzip3 support. https://github.com/kspalaiologos/bzip3 * lib/Automake/Options.pm (_is_valid_easy_option): likewise. * lib/am/distdir.am (dist-bzip3): likewise. * t/dist-formats.tap: likewise. * t/dist-bzip3.sh: new test. * t/list-of-tests.mk (handwritten_TESTS): add it. * NEWS: mention this. * THANKS: add Kamila.