testing: Switch to Python 3

Both swidGenerator and strongTNC were migrated to Python 3.
This allows to migrate all testing scripts to Python 3, too.

testing/scripts/recipes/013_strongswan.mk

@@ -129,6 +129,6 @@ build: configure
 
 install: build
 	cd $(BUILDDIR) && make -j install && \
-		cd $(DIR)/src/libcharon/plugins/vici/python && python setup.py install
+		cd $(DIR)/src/libcharon/plugins/vici/python && python3 setup.py install
 	# for Python-based updown scripts
-	pip install python-daemon
+	pip3 install python-daemon