Python: drop EOL versions (#33898)

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.

var/spack/repos/builtin/packages/dealii/package.py

@@ -182,10 +182,8 @@ class Dealii(CMakePackage, CudaPackage):
     depends_on("p4est", when="+p4est+mpi")
     depends_on("petsc+mpi~int64", when="+petsc+mpi~int64")
     depends_on("petsc+mpi+int64", when="+petsc+mpi+int64")
-    depends_on("petsc@:3.6.4", when="@:8.4.1+petsc+mpi")
     depends_on("scalapack", when="@9.0:+scalapack")
     depends_on("slepc", when="+slepc+petsc+mpi")
-    depends_on("slepc@:3.6.3", when="@:8.4.1+slepc+petsc+mpi")
     depends_on("slepc~arpack", when="+slepc+petsc+mpi+int64")
     depends_on("sundials@:3~pthread", when="@9.0:9.2+sundials")
     depends_on("sundials@5:5.8", when="@9.3:9.3.3+sundials")