ipython-7.17 stops support for python-3.6

The latest release of ipython-7.17 stops support for python-3.6 (https://github.com/ipython/ipython/blob/master/setup.py#L43). However, this change is not reflected in ipykernel, specifically at https://github.com/ipython/ipykernel/blob/master/setup.py#L89. We have an application relying on ipykernel, and we use `pip download` to get ipykernel and its dependencies. It downloads ipython-7.17, but the host only has python-3.6. So, things start to break.

While we could add a constraint to `pip download` to specify the specific version of ipython, should ipykernel be updated to make it clear python-3.6 is not compatible with the latest release of ipython (and the latest release of ipykernel as a result)?

@blink1073