HEP 1: Python version support

[ianthomas23]

This PR adds the first HoloViz Enhancement Proposal, on python version support, to the holoviz repo. It is placed in a new hep directory, and the idea is that future HEPs will be placed in the same directory and there will be one or more links from holoviz.org to them.

Rendered markdown is at https://github.com/holoviz/holoviz/blob/hep1/hep/hep1.md.

Everything here is open to discussion: the HEP itself, the format of the HEP, the directory structure, etc.

[droumis]

looks good. Might be good to mention this in the SC meeting next Friday April 28

[jbednar]

LGTM

[ianthomas23]

This has 2 approvals. I'll give people until Monday to express any contrary opinions, otherwise I will merge it myself.

[maximlt]

The document defines the policy for when a Python version should be dropped. All the HoloViz projects (attempt to) follow Semantic Versioning. I think that our convention for dropping support for a Python version was that it wouldn't happen in a patch version but at least in a minor version. Now, I'm not entirely sure that was the convention, and if it was, I'm not entirely sure we should keep following it. And really, I have no opinion on that, except that I think we should agree on what's acceptable and probably add it to that document?

[ianthomas23]

@maximlt I'm not familiar with the (possible) convention on relating project and python versions, either within HoloViz or outside. If there is a convention going forward then yes we should add a comment or link here about it. But probably the best approach would be for the HoloViz project versioning to be documented first, e.g. in HEP2, even if it is just a single paragraph linking to standard SemVer practice.