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/bootstrap.yml

@@ -214,7 +214,7 @@ jobs:
       - name: Bootstrap clingo
         run: |
           set -ex
-          for ver in '2.7' '3.6' '3.7' '3.8' '3.9' '3.10' ; do
+          for ver in '3.6' '3.7' '3.8' '3.9' '3.10' ; do
             not_found=1
             ver_dir="$(find $RUNNER_TOOL_CACHE/Python -wholename "*/${ver}.*/*/bin" | grep . || true)"
             echo "Testing $ver_dir"