Fix flake8 lint failures in tests/test_bseries.py

[Copilot]

CI lint was failing due to several style violations introduced in tests/test_bseries.py.

Changes

• F401 – Remove unused import math
• E305 – Add second blank line after _BT class definition
• E261 – Add required two spaces before inline comment on t4d assignment
• E221 – Remove extra alignment spaces before = on t7a–t7z variable definitions
• E241 – Remove extra spaces after , in _ORDER6_SIGMA_GAMMA and _ORDER7_SIGMA_GAMMA tuple lists
• E127 – Fix over-indented continuation line in @pytest.mark.parametrize decorator