python: change condition check for legacy opt for Python3

As I remember this worked.
But since `set -e` is set, I am a bit paranoid about it. In the sense that
it may fail if `ver` != 3.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>

lang/python/python-package-install.sh

@@ -77,8 +77,9 @@ if [ "$mode" == "sources" ] ; then
 	exit 0
 fi
 
-legacy=
-[ "$ver" == "3" ] && legacy="-b"
+if [ "$ver" == "3" ] ; then
+	legacy="-b"
+fi
 # default max recursion is 10
 max_recursion_level=20