pycompat: remove check for Python >= 3.6
We dropped support for Python 3.5 so this is always true.
Differential Revision: https://phab.mercurial-scm.org/D12363
hgdemandimport: delete check for Python 3.5
We dropped support for Python 3.5. So we no longer need to do this.
Differential Revision: https://phab.mercurial-scm.org/D12362
hg: always import hgdemandimport
The deleted if condition is always true now that we dropped Python 2
and 3.5.
Differential Revision: https://phab.mercurial-scm.org/D12361