tests: collapse elif PYTHON3 block
PYTHON3 is always True now so this logic should be identical as to before.
Differential Revision: https://phab.mercurial-scm.org/D12236
tests: collapse some more trivial if PYTHON3 blocks
This doesn't account for all of the references to PYTHON3. But it
accounts for the ones that are more trivial and don't entail logical
changes.
Differential Revision: https://phab.mercurial-scm.org/D12235
tests: require Python 3.5+ in run-tests.py
We change the version check logic to hard fail if running on
<= 3.5.0. The branch for <3.5 has been deleted. And the >=3.5
branch block has been dedented.
Differential Revision: https://phab.mercurial-scm.org/D12234