icefuzz: update Makefile to use $(PYTHON) variable
icefuzz: update Makefile to use $(PYTHON) variable

Don't hardcode `python3` as the name of the Python interpreter.

Signed-off-by: Sean Cross <d7e19930cc1f42c2d0781f4d9e6f1fe5891bf9cf@xobs.io>

SymbiFlow/icestorm,SymbiFlow/icestorm,SymbiFlow/icestorm,SymbiFlow/icestorm