github: Fix python build and checks on older Ubuntu releases

On 18.04, setuptools was apparently pulled in by python-pip but is not
by python3-pip and on Ubuntu 16.04 there is an issue with tox when
installed via pip3 (syntax error in one of the dependencies) and with
pip that dependency is not even available.

scripts/test.sh

@@ -178,9 +178,12 @@ all|coverage|sonarcloud)
 	if [ "$ID" = "ubuntu" -a "$VERSION_ID" = "20.04" ]; then
 		DEPS="$DEPS libiptc-dev"
 	else
-		DEPS="$DEPS iptables-dev"
+		DEPS="$DEPS iptables-dev python3-setuptools"
+	fi
+	# tox has dependencies that can't be resolved on 16.04 (even with pip)
+	if [ "$ID" != "ubuntu" -o "$VERSION_ID" != "16.04" ]; then
+		PYDEPS="tox"
 	fi
-	PYDEPS="tox"
 	if test "$1" = "build-deps"; then
 		if [ "$ID" != "ubuntu" -o "$VERSION_ID" != "16.04" ]; then
 			build_botan