Tornado 6.2 compatibility

[fjetter]

Tornado 6.2 was just released and we're hitting a few errors on CI. So far, the only thing I observed was a deprecation warning that is converted to an error. We should restore compatibility and remove the pinning introduced in #6668

cc @graingert

[jrbourbeau]

@gabicca noted in #6979 (comment) that package solve conflicts are showing up with Jupyter because the latest jupyter-clinet==7.3.5 release requires tornado >=6.2

[jrbourbeau]

Another jupyter-related report #6984

[crusaderky]

There are several failing tests; moving this back to in progress

[gabicca]

Hello, can you please give us an update on how this work is going? When can we expect distributed to be compatible with tornado==6.2?

[graingert]

Hello, can you please give us an update on how this work is going? When can we expect distributed to be compatible with tornado==6.2?

hopefully in the next release!