Switch to heap types using PyType_FromSpec

[cjdoris]

This PR implements the suggestion from @ZeroIntensity to switch from static PyTypeObject structures to heap-allocated types using PyType_FromSpec.

Changes

• Added PyType_Spec and PyType_Slot structs to C API bindings in src/C/consts.jl
• Added all Python type slot constants from ypeslots.h for proper slot definitions
• Converted PyJuliaBase_Type creation from manual PyTypeObject to PyType_FromSpec in src/JlWrap/C.jl
• Used weaklistoffset member definition for backward compatibility with older Python versions
• Fixed PyType_FromSpec function signature in src/C/pointers.jl

Key Benefits

• Avoids ABI compatibility issues by using slot-based type definitions instead of structure fields
• Future-proof approach that works with different Python versions
• Cleaner implementation using the modern Python C API

Implementation Details

The conversion replaces the old approach of manually filling a PyTypeObject struct with a modern PyType_FromSpec approach that:

1. Creates a PyType_Spec with basic type information
2. Defines an array of PyType_Slot structures for all type methods and protocols
3. Uses weaklistoffset member definition for weakref support (compatible with older Python versions)
4. Properly handles the buffer protocol through slots

Closes #641