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"

lib/spack/external/_vendoring/ruamel/yaml/scalarbool.py

@@ -0,0 +1,47 @@
+# coding: utf-8
+
+"""
+You cannot subclass bool, and this is necessary for round-tripping anchored
+bool values (and also if you want to preserve the original way of writing)
+
+bool.__bases__ is type 'int', so that is what is used as the basis for ScalarBoolean as well.
+
+You can use these in an if statement, but not when testing equivalence
+"""
+
+from ruamel.yaml.anchor import Anchor
+
+if False:  # MYPY
+    from typing import Text, Any, Dict, List  # NOQA
+
+__all__ = ['ScalarBoolean']
+
+
+class ScalarBoolean(int):
+    def __new__(cls, *args, **kw):
+        # type: (Any, Any, Any) -> Any
+        anchor = kw.pop('anchor', None)
+        b = int.__new__(cls, *args, **kw)
+        if anchor is not None:
+            b.yaml_set_anchor(anchor, always_dump=True)
+        return b
+
+    @property
+    def anchor(self):
+        # type: () -> Any
+        if not hasattr(self, Anchor.attrib):
+            setattr(self, Anchor.attrib, Anchor())
+        return getattr(self, Anchor.attrib)
+
+    def yaml_anchor(self, any=False):
+        # type: (bool) -> Any
+        if not hasattr(self, Anchor.attrib):
+            return None
+        if any or self.anchor.always_dump:
+            return self.anchor
+        return None
+
+    def yaml_set_anchor(self, value, always_dump=False):
+        # type: (Any, bool) -> None
+        self.anchor.value = value
+        self.anchor.always_dump = always_dump