Change the python invocation command in all the run script to python3

INSTALL.swift

examples/Cooling/CoolingBox/run.sh

examples/Cooling/CoolingHalo/makeIC.py

examples/Cooling/CoolingHalo/makeIC_random_box.py

examples/Cooling/CoolingHalo/run.sh

examples/Cooling/CoolingHaloWithSpin/makeIC.py

examples/Cooling/CoolingHaloWithSpin/makeIC_random_box.py

examples/Cooling/CoolingHaloWithSpin/run.sh

examples/Cooling/CoolingRates/README

examples/Cosmology/ComovingSodShock_1D/run.sh

examples/Cosmology/ComovingSodShock_2D/run.sh

examples/Cosmology/ComovingSodShock_3D/run.sh

examples/Cosmology/ZeldovichPancake_3D/run.sh

examples/EAGLE_low_z/EAGLE_6/testVELOCIraptor.sh

examples/GEAR/AgoraCosmo/README

examples/GravityTests/DiscPatch/GravityOnly/makeIC.py

examples/GravityTests/DiscPatch/GravityOnly/run.sh

examples/GravityTests/DiscPatch/HydroStatic/run.sh

examples/GravityTests/DiscPatch/HydroStatic_1D/run.sh

examples/GravityTests/ExternalPointMass/run.sh