[update_chassisdb_config] Convert to Python 3 (#5838)

- Convert update_chassisdb_config script to Python 3
- Reorganize imports per PEP8 standard
- Two blank lines precede functions per PEP8 standard

files/scripts/update_chassisdb_config

@@ -1,13 +1,15 @@
-#!/usr/bin/python
+#!/usr/bin/python3
+
+import argparse
 import json
 import os
 import syslog
-import argparse
 
 database_config_file = "/var/run/redis/sonic-db/database_config.json"
 redis_chassis = 'redis_chassis'
 chassis_db = 'CHASSIS_APP_DB'
 
+
 def main():
     parser = argparse.ArgumentParser(description=
           "Update chassis_db config from database-config.json")
@@ -63,5 +65,6 @@ def main():
     syslog.syslog(syslog.LOG_INFO,
        'remove chassis_db from config file {}'.format(jsonfile))
 
+
 if __name__ == "__main__":
     main()