HEP 1: Python version support (#362)

* HEP 1: Python version support

* Review comments

hep/README.md

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+# HoloViz Enhancement Proposals
+
+This directory contains HoloViz Enhancement Proposals (HEP) which document
+important features or processes that apply to all HoloViz Open Source projects.

hep/hep1.md

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+# HEP 1: Python version support
+
+This enhancement proposal describes the possible policy that HoloViz projects
+might adopt for support of Python versions. It is currently a discussion
+document and has not yet been accepted.
+
+## Background
+
+Historically a new version of Python has been released every one to two years,
+and in future annual releases are planned every October. The lifespan of each
+version is typically five years, so on any particular date there are expected
+to be five active Python versions. The current status of Python versions is
+illustrated in the
+[Python Developer's Guide](https://devguide.python.org/versions).
+
+NumPy have a more restrictive policy of supporting Python version released in
+the last 42 months (three and a half years), with a few extra caveats, in
+[NEP29](https://numpy.org/neps/nep-0029-deprecation_policy.html). They
+therefore support the latest three to four Python releases.
+
+A number of projects in the scientific Python ecosystem follow the NumPy policy
+including
+
+- [Matplotlib](https://matplotlib.org/stable/devel/min_dep_policy.html)
+- [numba](https://numba.readthedocs.io/en/stable/release-notes.html#version-0-47-0-jan-2-2020)
+- [pandas](https://pandas.pydata.org/pandas-docs/stable/development/policies.html)
+- [xarray](https://xarray.pydata.org/en/v2023.04.0/getting-started-guide/installing.html#minimum-dependency-versions)
+
+Bokeh, a library of particular importance to HoloViz, has a
+[policy](https://github.com/bokeh/bokeh/wiki/BEP-9:-Downstream-Version-Support)
+to support at least the latest three versions of Python so although it does not
+explicitly follow NumPy's policy it is similar.
+
+At the time of writing in mid April 2023, Python versions currently supported
+are 3.7 to 3.11. 3.7 reaches end of life at the end of June, and Python 3.12
+will be released in October. NumPy's previous release (1.24.2) supported Python
+3.8 to 3.11 and the next release will support 3.9 to 3.11. Bokeh's most recent
+release (3.1.0) supported Python 3.8 to 3.11, and the next release (3.2.0) will
+support Python 3.9 to 3.11.
+
+## Policy
+
+Ideally HoloViz projects should support as many Python releases as are
+reasonably possible. Support for a new Python release should ideally occur on
+the day of release, but the limiting factor here is often numba as that is
+usually delayed.
+
+Support for old versions of Python is limited, in a practical sense, by
+dependent libraries' support for old versions. Dependencies that have a slow
+change cadence are usually not a problem, but those with a fast change cadence
+(e.g. pyarrow) or with major backward-breaking changes (e.g. Bokeh 3.0, pandas
+2.0) potentially are. A desire to support older versions of Python here can
+result in HoloViz projects effectively performing the integration testing,
+fixes and workarounds for those dependencies that have already dropped support
+for older Python.
+
+In deciding on a policy for support of old Python releases, the HoloViz
+projects can be divided into three groups:
+
+1. Colorcet and Param: projects with few dependencies.
+2. Panel, Lumen, HoloViews, hvPlot and Datashader: Panel is tightly coupled to
+Bokeh, and the other projects here are commonly used in Panel so they should
+also be considered tightly coupled to Bokeh.
+3. GeoViews and SpatialPandas: projects with geospatial dependencies (install
+or CI).
+
+For 1, the limited dependencies means that support can be for all Python
+releases that are currently active.
+
+For 2, the tight coupling with Bokeh means that Bokeh's policy should be
+followed. However, this can be relaxed slightly so that following a Bokeh
+release that drops support for a particular Python version each of the HoloViz
+projects will make one further release supporting that Python version before
+dropping it themselves.
+
+For 3 the geospatial dependencies often cause problems by not being available
+as wheels on PyPI and being difficult to resolve on conda. The HoloViz policy
+should be the same as that of 2 above for uniformity.
+
+## Illustration
+
+To illustrate the proposed policy, this is what it means for the remainder of
+2023.
+
+All projects currently support Python 3.11 and will support Python 3.12 when it
+is released in October or as soon as possible after that.
+
+Colorcet and Param currently support Python 3.7 upwards. When this reaches end
+of life at the end of June, the minimum support Python version will increase to
+3.8 for their next releases.
+
+The current Bokeh release (3.1.0) supports minimum Python of 3.8 and the next
+release (3.2.0) will support minimum Python 3.9. All HoloViz projects (except
+Colorcet and Param which are covered above) should
+drop support for Python 3.7 as soon as possible to support a minimum Python of
+3.8. Following release of Bokeh 3.2.0 there should be one further release per
+HoloViz project that supports Python 3.8 and then the minimum Python version
+for each should increase to 3.9.
+
+## Revisions
+
+Changes to this document shall be recorded below:
+
+| Date	     | Change      |
+| ---------- | ----------- |
+| 2023-04-19 | First draft |