revset: support ranges in #generations relation

tests: fix module-import warnings in test-update-atomic.t These were caught by test-check-module-imports.t on Python 3. This should also make python3-buildbot green again. Differential Revision: https://phab.mercurial-scm.org/D5674

py3: use dict.items() instead of dict.iteritems() in tests/test-lfs-server.t dict.iteritems() is not present on Python 3. Differential Revision: https://phab.mercurial-scm.org/D5673

py3: add b'' prefixes in tests/test-lfs-serve.t # skip-blame because just b'' prefixes Differential Revision: https://phab.mercurial-scm.org/D5672