Change shebang used in CLI demo. (#7389)

Change from system Python to environment python3.  For Ubuntu 20.04, only `python3` is
available and there's no `python`.  So at least `python3` is consistent with Python
virtual env, Ubuntu and anaconda.

demo/CLI/yearpredMSD/csv2libsvm.py

@@ -1,3 +1,5 @@
+#!/usr/bin/env python3
+
 import sys
 fo = open(sys.argv[2], 'w')