clean up pyupgrade changes

doc/conf.py

examples/dagrt-fusion.py

examples/geometry.py

examples/maxwell/cavities.py

examples/wave/var-propagation-speed.py

examples/wave/wave-min-mpi.py

grudge/dt_finding.py

grudge/function_registry.py

grudge/models/__init__.py

grudge/models/advection.py

grudge/models/burgers.py

grudge/models/diffusion.py

grudge/models/gas_dynamics/__init__.py

grudge/models/gas_dynamics/lbm.py

grudge/models/nd_calculus.py

grudge/models/pml.py

grudge/models/poisson.py

grudge/models/second_order.py

grudge/models/wave.py

grudge/shortcuts.py