How to use the zulip.API_VERSTRING function in zulip

def get_recent_messages(client, narrow_str, count=100):
    # type: (zulip.Client, str, int) -> List[Dict[str, Any]]
    narrow = [word.split(':') for word in narrow_str.split()]
    req = {
        'narrow': narrow,
        'num_before': count,
        'num_after': 0,
        'anchor': 1000000000,
        'apply_markdown': False
    }
    old_messages = client.do_api_query(req, zulip.API_VERSTRING + 'messages', method='GET')
    if 'messages' not in old_messages:
        return []
    return old_messages['messages']

import sys
import os
import simplejson

sys.path.append(os.path.join(os.path.dirname(__file__), '..', 'api'))
import zulip

zulip_client = zulip.Client(site="http://localhost:9991", client="ZulipGithubPayloadSender/0.1")

payload_dir = "zerver/fixtures/github"
for filename in os.listdir(payload_dir):
    with open(os.path.join(payload_dir, filename)) as f:
        req = simplejson.loads(f.read())
    req['api-key'] = zulip_client.api_key
    req['email'] = zulip_client.email
    zulip_client.do_api_query(req, zulip.API_VERSTRING + "external/github")