run pyupgrade --py36-plus

examples/python/call-external.py

loopy/auto_test.py

loopy/check.py

loopy/kernel/creation.py

loopy/kernel/function_interface.py

loopy/kernel/tools.py

loopy/library/function.py

loopy/library/reduction.py

loopy/preprocess.py

loopy/program.py

loopy/statistics.py

loopy/symbolic.py

loopy/target/c/__init__.py

loopy/target/c/compyte

loopy/target/cuda.py

loopy/target/opencl.py

loopy/target/pyopencl.py

loopy/target/python.py

loopy/transform/callable.py

loopy/transform/diff.py