How to run a systemd service script as a different user

I am trying to run a systemd service as a different user (db2inst1) which looks as follows:

[Unit]
Description=Dummy Service
After=multi-user.target
Conflicts=getty@tty1.service

[Service]
Type=simple
User=db2inst1
Group=db2iadm1
ExecStart=/usr/bin/python3 /usr/bin/dummy_service.py
StandardInput=tty-force

[Install]
WantedBy=multi-user.target

dummy_service.py looks as follows:

import logging
import random
import time
from systemd.journal import JournaldLogHandler

# get an instance of the logger object this module will use
logger = logging.getLogger(__name__)

# instantiate the JournaldLogHandler to hook into systemd
journald_handler = JournaldLogHandler()

# set a formatter to include the level name
journald_handler.setFormatter(logging.Formatter(
    '[%(levelname)s] %(message)s'
))

# add the journald handler to the current logger
logger.addHandler(journald_handler)

# optionally set the logging level
logger.setLevel(logging.DEBUG)

if __name__ == '__main__':
    while True:
        # log a sample event
        logger.info(
            'test log event to systemd! Random number: %s',
            random.randint(0, 10)
        )

        # sleep for some time to not saturate the journal
        time.sleep(5)

But when I start the service I keep getting:

[root@dashmpp-head-0 - Db2wh /]# systemctl status dummy.service
● segment.service - Segment Service
   Loaded: loaded (/usr/lib/systemd/system/dummy.service; disabled; vendor preset: disabled)
   Active: failed (Result: exit-code) since Mon 2020-09-21 21:16:37 UTC; 8s ago
  Process: 241156 ExecStart=/usr/bin/python3 /usr/bin/dummy_service.py (code=exited, status=208/STDIN)
 Main PID: 241156 (code=exited, status=208/STDIN)

Sep 21 21:16:37 dashmpp-head-0 systemd[1]: Started Dummy Service.
Sep 21 21:16:37 dashmpp-head-0 systemd[1]: dummy.service: main process exited, code=exited, status=208/STDIN
Sep 21 21:16:37 dashmpp-head-0 systemd[1]: Unit dummy.service entered failed state.
Sep 21 21:16:37 dashmpp-head-0 systemd[1]: dummy.service failed.

If run the python script as root user in dummy.service file it works fine. Any pointers why I am not able to execute the python script as different user?

I am on Centos7.