`#[pyclass]` trait ergonomics

Following up to #4202 and #4206 I wanted to write down something I've been thinking about for a while. It looks like we're moving ahead with accepting those options so I'll assume that at least the design choice to have these options is accepted.

In summary, I think there are at least five convenience options to add to #[pyclass] to automatically expose Python magic methods based on Rust trait implementations:

• #[pyclass(eq)] for equality based off PartialEq - #4210
• #[pyclass(ord)] for ordering operators based off PartialOrd - #4202
• #[pyclass(hash)] for hash based off Hash - #4206
• #[pyclass(str)] for __str__ based off Display. I think we could also have #[pyclass(str = "<format>")] to specify a format-args-compatible string concisely.
• #[pyclass(repr)] for __repr__ based off Debug. Similarly to the above I think we could have #[pyclass(repr = "<format>")].

Based off the recent PRs I think the basic implementation of these is more or less agreed upon, possibly excluding the extra argument on the string formatting options.

As we proceed with / make progress on the above, I think there are two further steps which make sense to explore:

1. Simple enums currently generate eq and repr automatically. I think we should be deprecating the automatic generation and start requiring opt-in to the new behaviour, to be consistent with everything else. I think it's relatively easy to do this as we just need to emit the deprecation warning if the eq or repr arguments weren't passed.
2. I think a future extension would be to have #[pyclass(init)] to automatically generate a constructor (like we already do for complex enums) and then #[pyclass(dataclass)] which does some or all of the above conveniences (maybe depending on whether the class is frozen).

Start by reading the related issues #4202, #4206, and #4210 to understand the accepted #[pyclass] option design. The remaining proposal concerns repr, deprecating automatic enum behavior, and possible init/dataclass extensions; work is done only after the scope and unresolved formatting choices are agreed.