Upgrade PyYAML to version 6 (#6183)

* Fix load_all() argument
* Remove Python 2 remnants
  + Replace Text with str
  + Replace alias "_Str" with str
  + Import from collections.abc and re where applicable
  + Remove Python 2 branches
* Fix PyYaml allowlist entries
  + Add yaml._yaml and move CParser and CEmitter there.
  + Add missing functions, classes, and arguments.
* Use relative imports in some modules.
* Add __all__ to yaml.cyaml.
* Remove unnecessary noqa markers.

stubs/PyYAML/@tests/stubtest_allowlist.txt

@@ -1,8 +0,0 @@
-yaml.CBaseDumper.__init__
-yaml.CDumper.__init__
-yaml.CEmitter
-yaml.CParser
-yaml.YAMLObjectMetaclass.__init__
-yaml.constructor.FullConstructor.set_python_instance_state
-yaml.cyaml.CBaseDumper.__init__
-yaml.cyaml.CDumper.__init__