Python core: deep-copy review + matplotlib threading fix

[sgrimbly]

Two long-standing Python-side TODOs that affect parity work:

• src/Python/math_utils.py:60 — confirm whether deep-copying is required, or whether A and B are already passed by reference safely. Tag: #testing.
• src/Python/display.py:5 — fix the potential threading / halting behaviour of Matplotlib rendering during code execution. Tag: #visualisation #bug.

Both are author-signed (St John), so likely real issues, not scratch notes.