Fix issue where Jupyter Notebooks in ArcGIS Pro will not open if using an http(s)_proxy.

[MichaelPatton_CDFW]

Currently, Jupyter notebooks will not open in ArcGIS Pro if you are using a http_proxy environment variable at the system or user level. Many users are required to use a proxy setting due to department policies and therefore can no longer use notebooks. 

[RTPL_AU]

I suspect this is a network level config issue - with the proxy or with another level of security that impacts the interaction between the notebook server and client components. 
Can you test and run another web service on the local machine? Something portable like XAMPP is handy for this.

Edit:

Both ArcGISPro.exe and python.exe open a number of listening ports (on 127.0.0.1 rather than 0.0.0.0 fwiw)  when you open /add a notebook. If the proxy is a consume all rather than just for specific web service ports / ips you'll have issues.
I don't think this is an Idea - rather work with IT and Esri to figure out the correct configuration.

I've had similar issues when doing things over some VPNs.