Remove support for running with Python 2.7 (#33063)

* Remove CI jobs related to Python 2.7

* Remove Python 2.7 specific code from Spack core

* Remove externals for Python 2 only

* Remove llnl.util.compat

.github/workflows/ci.yaml

@@ -20,12 +20,6 @@ jobs:
     uses: ./.github/workflows/valid-style.yml
     with:
       with_coverage: ${{ needs.changes.outputs.core }}
-  audit-ancient-python:
-    uses: ./.github/workflows/audit.yaml
-    needs: [ changes ]
-    with:
-      with_coverage: ${{ needs.changes.outputs.core }}
-      python_version: 2.7
   all-prechecks:
     needs: [ prechecks ]
     runs-on: ubuntu-latest
@@ -85,7 +79,7 @@ jobs:
     needs: [ prechecks ]
     uses: ./.github/workflows/windows_python.yml
   all:
-    needs: [ windows, unit-tests, bootstrap, audit-ancient-python ]
+    needs: [ windows, unit-tests, bootstrap ]
     runs-on: ubuntu-latest
     steps:
     - name: Success