demo: modify shebang to call Python through /usr/bin/env

.gitignore

@@ -5,3 +5,4 @@
 /build/
 /dist/
 /re6stnet.egg-info/
+.idea
\ No newline at end of file

demo/demo

-#!/usr/bin/python
+#!/usr/bin/env python3
 import argparse, math, nemu, os, re, signal
 import socket, sqlite3, subprocess, sys, time, weakref
 from collections import defaultdict

demo/py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 def __file__():
     import argparse, os, sys
     sys.dont_write_bytecode = True