Fix monkeypatch style and remove unused sys import in tests

[tbjers]

Summary

• Replace runtime sys.modules[Packer.__module__] lookups in monkeypatch.setattr calls with the idiomatic pytest dotted-string form ("openscad_packer.packer.getASTfromString"), which patches the same binding more readably
• Remove the now-unused import sys that was only needed by the old form

Test plan

• uv run pytest tests/test_packer.py — all 61 tests pass

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.59%. Comparing base (28cd26e) to head (9a5faf0).

Additional details and impacted files

@@           Coverage Diff           @@
##             main       #8   +/-   ##
=======================================
  Coverage   99.59%   99.59%           
=======================================
  Files           6        6           
  Lines         244      244           
  Branches       53       53           
=======================================
  Hits          243      243           
  Partials        1        1           

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