[Python] Fix type annotations for protocols, ABCs, Atom, and Set module

[dbrattli]

Summary

• Separated protocols from ABC base classes for cleaner inheritance architecture
• Added proper Atom[T] type for module-level mutable values
• Created Python-specific Set.fs to avoid JS-specific constructs
• Fixed protocol signatures and array stub types for Pyright compatibility
• Added cast(type, None) for null assignments to typed variables

Changes

Compiler (src/Fable.Transforms/Python/)

• Fable2Python.Annotation.fs: Added Atom[T] type annotation support; fixed protocol type mappings
• Fable2Python.Transforms.fs: Map IDisposable → DisposableBase, IEnumerator_1 → EnumeratorBase for inheritance; wrap Null t with cast(type, None)
• Replacements.fs: Added makeAtomType for proper Atom[T] return type from create_atom

Library (src/fable-library-py/)

• protocols.py (new): Consolidated pure protocol definitions (structural subtyping only)
• util.py: Added ABC base classes (DisposableBase, EnumeratorBase, EnumerableBase) with implementations; uses Self return type for __enter__ methods
• Set.fs (new): Python-specific Set module without JS-specific interfaces
• core/array.pyi: Fixed equals_with to accept None; fixed index return types to Int32
• choice.pyi, option.pyi: Updated protocol signatures

Other

• Fable.Library.Python.fsproj: Use local Set.fs instead of TypeScript version
• event.py, observable.py: Use DisposableBase.create() instead of IDisposable.create()

(This brings us down to 945 type annotation errors)