Remove dead code supporting old versions of Julia/Python

[cjdoris]

This PR removes dead code that was supporting old versions of Julia and Python:

Changes:

• Remove fallback to Py_Finalize() for Python < 3.6, always use Py_FinalizeEx()
• Update minimum Python version check from 3.5+ to 3.9+ (aligns with recent version bump)
• Remove Py_Finalize function signature, keeping only Py_FinalizeEx
• Simplify pointer initialization by removing conditional logic
• Remove Julia version check for stacktrace formatting (< 1.6.0), keep only modern formatting

Files changed:

• src/C/context.jl: Simplified finalization and version checking
• src/C/pointers.jl: Removed legacy function signatures
• src/Core/err.jl: Removed legacy stacktrace formatting

Net reduction of 18 lines, making the codebase cleaner by removing compatibility shims that are no longer needed.