Recent CI Flake8 failure

[garrettwrong]

Appears something in Flake8 has upgraded and our tox check is failing in CI.

check run-test: commands[0] | flake8 .
./src/aspire/covariance/covar2d.py:353:62: B023 Function definition does not bind loop variable 'S'.
./src/aspire/covariance/covar2d.py:354:63: B023 Function definition does not bind loop variable 'A_ell'.
./src/aspire/covariance/covar2d.py:661:62: B023 Function definition does not bind loop variable 'S'.
./src/aspire/covariance/covar2d.py:662:63: B023 Function definition does not bind loop variable 'A_ell'.

Patch should follow.