Enable F841 unused-variable lint, fix

contrib/c-integer-semantics.py

loopy/kernel/creation.py

loopy/kernel/function_interface.py

loopy/kernel/tools.py

loopy/library/reduction.py

loopy/preprocess.py

loopy/statistics.py

loopy/symbolic.py

loopy/target/c/__init__.py

loopy/target/c/c_execution.py

loopy/target/cuda.py

loopy/target/ispc.py

loopy/target/opencl.py

loopy/tools.py

loopy/transform/callable.py

loopy/type_inference.py

proto-tests/test_sem.py

proto-tests/test_sem_tim.py

proto-tests/test_tim.py

pyproject.toml