fix(msgpack): raise DecodeError for unhashable map keys - #1209

Entries for msgspec#1194, msgspec#1196, msgspec#1197, msgspec#1199 and msgspec#1209 are missing from the
changelog of the 0.22.0 release. Of the changes merged after msgspec#1211
moved the unreleased entries into the 0.22.0 section, only msgspec#1207 added
its own entry. This adds the missing entries next to the related
entries in that section, and marks msgspec#1196 as a breaking change: objects
that expose `__dataclass_fields__` only through instance attribute
access are no longer encoded as dataclasses.

Co-authored-by: Tseluiko Aleksandr <[email protected]>

`msgspec.json.decode`, `msgspec.msgpack.decode` and `msgspec.convert` raise a
bare `TypeError` when a value can't be added to a `set`/`frozenset`:

    >>> msgspec.json.decode(b'[{"a": 1}]', type=set)
    TypeError: unhashable type: 'dict'

`DecodeError` is documented as "An error occurred while decoding an object",
so `except msgspec.DecodeError` doesn't catch this. The same bug was fixed for
msgpack map keys in msgspec#1209; the set item paths still leak it.

`mpack_decode_set`, `json_decode_set` and `convert_seq_to_set` now translate
the `TypeError` raised by `PySet_Add` into a `ValidationError` (a subclass of
`DecodeError`) that points at the offending item:

    >>> msgspec.json.decode(b'[{"a": 1}]', type=set)
    msgspec.ValidationError: Expected a hashable value, got `dict` - at `$[0]`

`convert_dict_to_dict` leaks it the same way when a key converts to an
unhashable value, so the `PyDict_SetItem` call there gets the same treatment:

    >>> msgspec.convert({(1, 2): 0}, dict[list[int], int])
    msgspec.ValidationError: Expected a hashable value, got `list` - at `key` in `$`

Note the msgpack decoder decodes arrays as tuples when a hashable value is
required, so arrays still decode into sets there; only maps, `Ext` values and
unhashable `dec_hook` results error out.

Tests: all 19 new cases raise a bare `TypeError` without the fix.

- `tests/unit/test_common.py`: json and msgpack, a `set`/`frozenset` item and a
  nested `$.values[0]` path.
- `tests/unit/test_json.py`: `[[1, 2]]` decodes as a list, which can't be added
  to a `set`, `frozenset` or `set[list[int]]`.
- `tests/unit/test_msgpack.py`: a map and an `Ext` value as set items.
- `tests/unit/test_convert.py`: `convert` with a dict/list item, a struct field
  path, and an unhashable dict key (4 cases, 3 of them parametrized over
  `dict`/`dict` subclass/duck mapping).

`pytest tests/unit` (Python 3.11, Windows): 6549 passed, 243 skipped, 2 errors.
Both errors are `test_cpylint.py` reading `_core.c` with the local cp936
default encoding, and they fail identically on `main`.
`ruff check`, `ruff format --diff` and `codespell` pass.

Signed-off-by: simpleqt <[email protected]>