Mapping can be a key in YAML

[sergio-nsk]

The very similar issue PyYaml can't parse mappings with lists as keys #339 The valid YAML

{1: 1}: first

is expected to be loaded into a dictionary, for example

{frozenset({(1, 1)}): 'first'}  # {frozenset({1: 1}.items()): 'first'}

but results in the error

>>> p=yaml.load('{1: 1}: first')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Python310\lib\site-packages\yaml\__init__.py", line 114, in load
    return loader.get_single_data()
  File "C:\Python310\lib\site-packages\yaml\constructor.py", line 51, in get_single_data
    return self.construct_document(node)
  File "C:\Python310\lib\site-packages\yaml\constructor.py", line 60, in construct_document
    for dummy in generator:
  File "C:\Python310\lib\site-packages\yaml\constructor.py", line 413, in construct_yaml_map
    value = self.construct_mapping(node)
  File "C:\Python310\lib\site-packages\yaml\constructor.py", line 218, in construct_mapping
    return super().construct_mapping(node, deep=deep)
  File "C:\Python310\lib\site-packages\yaml\constructor.py", line 141, in construct_mapping
    raise ConstructorError("while constructing a mapping", node.start_mark,
yaml.constructor.ConstructorError: while constructing a mapping
found unhashable key
  in "<unicode string>", line 1, column 1:
    {1: 1}: first
    ^
>>>

[perlpunk]

related to https://github.com/yaml/pyyaml/issues/339