make/devices.cmake: added PYTHON3 property
make/devices.cmake: added PYTHON3 property

This is necessary because when using add_target_fpga_boards the python3
env var is not set and the dependencies are not met, hence the
compilation fails because of missing path when importing python libs

Signed-off-by: Alessandro Comodi <622632e70fd6771cf2cd943f69e5e51ab66adef8@antmicro.com>

SymbiFlow/symbiflow-arch-defs,SymbiFlow/symbiflow-arch-defs