[configlet] Python3 compatible syntax for extracting a key from the dict (sonic-net#1721)

Signed-off-by: Neetha John <[email protected]>

Syntax error seen while trying to apply a configlet
Traceback (most recent call last):
File "/usr/local/bin/configlet", line 212, in
main()
File "/usr/local/bin/configlet", line 205, in main
process_entry (do_update, i)
File "/usr/local/bin/configlet", line 175, in process_entry
do_operate(op_upd, t, (), data[t])
File "/usr/local/bin/configlet", line 162, in do_operate
if type(lst[lst.keys()[0]]) == dict:
TypeError: 'dict_keys' object is not subscriptable

What I did
Use next(iter()) to derive a key from the dict. This is both python2 and python3 compatible

How to verify it
Tried applying the json file below using the command 'configlet -j test.json -d'. Configlet was applied successfully. No errors were seen
[
{
"ACL_TABLE": {
"EVERFLOW": {},
"EVERFLOWV6": {}
}
}
]

Author: neethajohn

scripts/configlet

@@ -159,7 +159,7 @@ def do_delete(t, k, lst):
 
 def do_operate(op_upd, t, k, lst):
     if lst:
-        if type(lst[lst.keys()[0]]) == dict:
+        if type(lst[next(iter(lst))]) == dict:
             for i in lst:
                 do_operate(op_upd, t, k+(i,), lst[i])
             return