build: change PYTHON to python3

On recent macOS, /usr/bin/python3 is a wrapper that finds the right python executable
It checks argv[0] to determine if python2 or python3 should be called. Always execute
it as python3 to ensure it calls the right version

Signed-off-by: Felix Fietkau <nbd@nbd.name>

rules.mk

@@ -274,7 +274,7 @@ BASH:=bash
 TAR:=tar
 FIND:=find
 PATCH:=patch
-PYTHON:=python
+PYTHON:=python3
 
 INSTALL_BIN:=install -m0755
 INSTALL_SUID:=install -m4755