Add notes regarding Python EOL

doc/source/guidelines/index.rst

@@ -14,6 +14,7 @@ Table of Contents
    :maxdepth: 2
 
    dev_practices
+   version_support
    app_interface_abstraction
    data_transfer_and_representation
    logging

doc/source/guidelines/version_support.rst

@@ -0,0 +1,75 @@
+Python Version Support
+======================
+
+When creating Python libraries, plan on supporting the oldest actively supported
+version of Python. For a quick reference, visit `Status of Python Branches
+<https://devguide.python.org/#status-of-python-branches>`_. Here is a summary as
+of 2022:
+
++---------+-------------+-----------------------+
+| Version | Released    | Security Support Ends |
++---------+-------------+-----------------------+
+| 3.10    | 04 Oct 2021 | 04 Oct 2026           |
++---------+-------------+-----------------------+
+| 3.9     | 05 Oct 2020 | 05 Oct 2025           |
++---------+-------------+-----------------------+
+| 3.8     | 14 Oct 2019 | 14 Oct 2024           |
++---------+-------------+-----------------------+
+| 3.7     | 27 Jun 2018 | 27 Jun 2023           |
++---------+-------------+-----------------------+
+
+Expect these to be the most commonly used Python versions. Note that some
+libraries like `NumPy <https://numpy.org/>`_ drop support for older versions of
+Python earlier than their end of life (EOL) as outlined in `NEP 29
+<https://numpy.org/neps/nep-0029-deprecation_policy.html#support-table>`_.
+
+Realize that users can still install the older version from PyPI via ``pip`` as
+the package manager (``pip``). When ``pip`` is used, it downloads and installs the most recent version
+of THE library that supports your version of Python. You can enforce a minimum-
+required Python version within ``setup.py`` with:
+
+.. code:: python
+
+   from setuptools import setup
+
+   [...]
+
+   setup(name="my_package_name",
+         python_requires='>3.6',
+   [...]
+   )
+
+This helps ``pip`` to know which versions of your library
+support which versions of Python. You can also impose an upper limit if you're
+sure you don't support certain versions of Python. For example, if you only
+support Python 3.6 through 3.9: ``python_requires='>=3.6, <3.10'``.
+
+
+Verifying Support
+-----------------
+The best way to validate whether a Python library supports a version of
+Python is to validate it via unit testing within CI/CD. An example
+GitHub workflow testing Python 3.6 through Python 3.10 on Windows and Linux
+would would start with::
+
+   jobs:
+     unittest:
+       name: Unit Testing
+       runs-on: ${{ matrix.os }}
+       strategy:
+         matrix:
+           os: [windows-latest, ubuntu-latest]
+           python-version: ['3.7', '3.8', '3.9', '3.10']
+
+       steps:
+         - uses: actions/checkout@v2
+
+         - name: Set up Python ${{ matrix.python-version }}
+           uses: actions/setup-python@v2
+           with:
+             python-version: ${{ matrix.python-version }}
+
+         - name: Unit testing
+           run: |
+             ...
+