Fixed all flake8 and black formatting issues to ensure clean CI runs.

Changes:
- Applied black formatting to 4 files
- Removed unused sys import in test_mixing_formula.py
- Removed duplicate imports in test files
- Fixed f-strings without placeholders (F541)
- All 63 tests passing
- All flake8 checks passing
- All black formatting checks passing

CI Status:
✅ Critical errors: 0
✅ Flake8 warnings: 0
✅ Black formatting: PASSED
✅ Tests: 63 passed, 0 failed