This PR deprecates using Spack to install [EOL Python versions](https://endoflife.date/python), including Python 2.7, 3.1–3.6. It also deprecates running Spack with Python 2.7. Going forward, we expect Spack to have the following Python support timeline.
### Running Spack
* Spack 0.18 (spring 2022): Python 2.7, 3.5–3.10
* Spack 0.19 (fall 2022): Python 2.7 (deprecated), 3.6–3.11
* Spack 0.20 (spring 2023): Python 3.6–3.11
### Building with Spack
* Spack 0.18 (spring 2022): Python 2.7, 3.1–3.10
* Spack 0.19 (fall 2022): Python 2.7, 3.1–3.6 (deprecated), 3.7–3.11
* Spack 0.20 (spring 2023): Python 3.7–3.11
This is a reboot of #28003. See #31824 for a detailed discussion of the motivation for this PR.
If you have concerns about this change, please comment on #31824.
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