tests: delete some no-py3 blocks And drop some nearby avoidable py3 checks in close proximity while we are here. Differential Revision: https://phab.mercurial-scm.org/D12257

tests: remove __future__ import from test-debugcommands.t This one is slightly more involved since it affects test output. Differential Revision: https://phab.mercurial-scm.org/D12256

tests: remove from __future__ from inline Python in tests This is no longer required since we require Python 3 and the linter no longer requires these statements. Differential Revision: https://phab.mercurial-scm.org/D12255