This PR removes [end of life](https://endoflife.date/python) versions of Python from Spack. Specifically, this includes all versions of Python older than 3.7.
See https://github.com/spack/spack/discussions/31824 for rationale. Deprecated in #32615. And #28003.
For anyone using software that relies on Python 2, you have a few options:
* Upgrade the software to support Python 3. The `3to2` tool may get you most of the way there, although more complex libraries may need manual tweaking.
* Add Python 2 as an [external package](https://spack.readthedocs.io/en/latest/build_settings.html#external-packages). Many Python libraries do not support Python 2, but you may be able to add older versions that did once upon a time.
* Use Spack 0.19. Spack 0.19 is the last release to officially support Python 3.6 and older
* Create and maintain your own [custom repository](https://spack.readthedocs.io/en/latest/repositories.html). Basically, you would need a package for Python 2 and any other Python 2-specific libraries you need.
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