InlineGUI server extension not compatible with Tornado 6

[jgosmann]

If Tornado 6 (instead of 5) is installed the server extension for the inline GUI fails to load.

[jgosmann]

The problem is that Tornado 6 removed the @web.asynchronous decorator. Instead co-routines (i.e. the async keyword) should be used. These only have been introduced with Python 3.4. Thus supporting Tornado 6 and Python versions < 3.4 might be complicated as the async keyword will be a syntax error in earlier versions.

As sort of a quick "fix", Nengo GUI could require tornado<6. But in general, a proper fix introducing Tornado 6 compatibility would be desirable). So, is Python 2.7 support still required? (It will soon be retired anyways.) Or would it be acceptable to require Python >=3.4 if one want to use the Jupyter integration?