Allow frozenset in attributes parameter of clean function

[ashrub-holvi]

Now it's dict[str, set[str]], and attempt to use frozenset will return

TypeError: argument 'attributes': 'frozenset' object cannot be converted to 'PySet'

but IMO using frozenset is a good practice, because if data is immutable good to use immutable type, for example allowed attributes can be defined in configuration and it's safer to use immutable type.

[ashrub-holvi]

also MappingProxyType can be allowed instead of dictionary

[adamchainz]

The TypeError comes from PyO3. It’s not clear to me from the PyO3 arg docs if there’s an easy way to allow all kinds of mappings or sets and have PyO3 convert them.

[messense]

Adding support for frozenset in PyO3/pyo3#3632

[ashrub-holvi]

Thank you! Do you think it's possible to add support of MappingProxyType? As I understand it's only way to have read-only dictionaries without adding third-party libraries.