PEP 489 Compatibility

[davidhewitt]

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Inspired by https://github.com/PyO3/pyo3/pull/2269#issuecomment-1087901951, this is an issue to track all of the places which we know need to change for full PEP 489 compatibility (draft implementation in #2245).

• Rust statics - #[pyclass] type objects, most implementations of PyTypeInfo, intern!

[davidhewitt]

The above is currently incomplete - please post / edit things as you think of them

[messense]

Linking some related PEPs here:

• PEP 489 – Multi-phase extension module initialization
• PEP 573 – Module State Access from C Extension Methods
• PEP 630 – Isolating Extension Modules
• PEP 684 - A Per-Interpreter GIL
• PEP 687 – Isolating modules in the standard library

[davidhewitt]

Beginning to think that this is something that we may not want to do for now - see https://github.com/PyO3/pyo3/discussions/2346#discussioncomment-2678241