pycompat: remove json.loads polyfill for Python 3.5
We no longer support Python 3.5 so this can be deleted.
Differential Revision: https://phab.mercurial-scm.org/D12364
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