Not sure if this is the right place to ask, but would it be possible to install emacs-25 on the bastion hosts? Apparently, the python mode in emacs-25 knows about the new python-3.5 asyncio keywords (async def, etc), but emacs-24 doesn't.
One of these looks like the right thing:
emacs25-lucid/oldstable,oldstable 25.1+1-4+deb9u1 amd64 GNU Emacs editor (with Lucid GUI support)
emacs25-nox/oldstable,oldstable 25.1+1-4+deb9u1 amd64 GNU Emacs editor (without GUI support)