Update vendored ruamel.yaml to v0.17.21 (#37008)

* Vendor ruamel.yaml v0.17.21

* Add unit test for whitespace regression

* Add an abstraction layer in Spack to wrap ruamel.yaml

All YAML operations are routed through spack.util.spack_yaml

The custom classes have been adapted to the new ruamel.yaml
class hierarchy.

Fixed line annotation issue in "spack config blame"

pyproject.toml

@@ -222,6 +222,9 @@ drop = [
   '^pygments/lexers/(?!python|__init__|_mapping).*\.py$',
   # trim rich's markdown support
   "rich/markdown.py",
+  # ruamel.yaml installs unneded files
+  "ruamel.*.pth",
+  "pvectorc.*.so"
 ]
 
 [tool.vendoring.typing-stubs]