Support Bokeh 2.1

[mrocklin]

The create_hosts_whitelist function was renamed to create_hosts_allowlist

Fixes #3741

FYI @bryevdv

[mrocklin]

CI passes. I don't know why the travis mark isn't green (it's green if you navigate to the page itself). Merging this quickly in order to get CI passing again.

[jacobtomlinson]

Given that doing a naive install of Dask like conda create -n newenv python=3.7 dask will result in the dashboard not working should we consider pushing a 2.18.1 release of distributed which just pins bokeh to 2.0 (i.e branch from last tag and change requirements only)?

For completeness you can do an install like conda create -n newenv python=3.7 dask bokeh=2.0and you'll be fine, but I just wasted 30 minutes working that out.

cc @mrocklin @jrbourbeau

[mrocklin]

@jrbourbeau any interest in firing off a 2.18.2 release today?