Mark a test xfail on Python 2.7 (#32526)

refers #32470

lib/spack/spack/test/concretize.py

@@ -1769,6 +1769,9 @@ def test_git_ref_version_errors_if_unknown_version(self, git_ref):
             s.concretized()
 
     @pytest.mark.regression("31484")
+    @pytest.mark.skipif(
+        sys.version_info[:2] == (2, 7), reason="Fixture fails intermittently with Python 2.7"
+    )
     def test_installed_externals_are_reused(self, mutable_database, repo_with_changing_recipe):
         """Test that external specs that are in the DB can be reused."""
         if spack.config.get("config:concretizer") == "original":