REST Call using 'requests' library in Python 2.7 - python-2.7

Question

I am trying to execute a REST POST call using 'request' library, and i am using python 2.7
What happening is, somehow the call is not getting executed via python script and and i am getting HTTPS connection error saying cannot connect to proxy.
I have verified in my system, proxy settings has no issue. In the same setup i am able to execute the same REST call using POSTMAN client & cURL.
Below is code snippet: (here the 'hostname' is private IP address and accessible internally only)
import requests
import json
import pprint
import sys
url = "https://<hostname>/rest/login-sessions"
json_data = {'authLoginDomain':'',
'password':'abc',
'userName':'xyz',
'loginMsgAck':'true'}
reqHeaders = {'Content-type':'application/json',
'X-Api-Version':'200'}
try:
reqPost = requests.post(url,data=json_data,headers=reqHeaders,verify = False)
pprint.pprint(resPost.json())
except requests.exceptions.Timeout:
print "Re-Try Again"
except requests.exceptions.TooManyRedirects:
print "Try with a different URL"
except requests.exceptions.RequestException as e:
print e
sys.exit(1)
Error code pasted below:
HTTPSConnectionPool(host='<hostIP>', port=443): Max retries exceeded with url: /rest/login-sessions (Caused by ProxyError('Cannot connect to proxy.', error('Tunnel connection failed: 503 Service Unavailable',)))
I tried finding solutions available in the web but none of them are working for me. I am confuse because when i am trying to execute the REST call via a python script then only it happening, via cURL & Postman i am able to do the same.
Please suggest if i am doing some mistake here OR any better way available to this.
I want to do it with python 'requests' library only. Also, i tried executing it with cURL using python subprocess, there also i am getting the same error.