[CHANGE] Network access OUT OF Sage Cells

[Rob Beezer]

The bad actors are ruining it for the rest of us. Again. :-(

Accessing the network with commands inside of a Sage Cell is going to be
severely restricted. Note that this means, for example, loading a data file
from a network address, as part of code inside a Sage Cell, will not function.

Here is the announcement from the (independent) Sage Cell project:

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https://groups.google.com/u/1/g/sage-cell/c/ETeONQMYbQM/m/Tzl-rFmkAQAJ

Unfortunately, due to some people abusing SageMathCell servers, access to the
Internet from cells is going to be removed or severely limited.

If you really need such access, your options are:
- maintain your own server, e.g. behind a campus firewall
- use CoCalc, where you can enable such access
- explain exactly what kind of access you need and why - perhaps we will figure
something out, but no guarantees on how long it will take, if ever