pylibfdt: Convert to Python 3

Build this swig module with Python 3.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/scripts/dtc/pylibfdt/Makefile b/scripts/dtc/pylibfdt/Makefile
index 15e66ad..42342c7 100644
--- a/scripts/dtc/pylibfdt/Makefile
+++ b/scripts/dtc/pylibfdt/Makefile
@@ -21,7 +21,7 @@
 		CPPFLAGS="$(HOSTCFLAGS) -I$(LIBFDT_srcdir)" OBJDIR=$(obj) \
 		SOURCES="$(PYLIBFDT_srcs)" \
 		SWIG_OPTS="-I$(LIBFDT_srcdir) -I$(LIBFDT_srcdir)/.." \
-		$(PYTHON2) $< --quiet build_ext --inplace
+		$(PYTHON3) $< --quiet build_ext --inplace
 
 $(obj)/_libfdt.so: $(src)/setup.py $(PYLIBFDT_srcs) FORCE
 	$(call if_changed,pymod)