py2: drop some more from __future__ statements
These are no longer needed after dropping support for Python 2.
Differential Revision: https://phab.mercurial-scm.org/D12258
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