Thank you Mark Sapiro.
I have tried as per your suggestion but failed unfortunately might be because of my improper setup.
But I learnt one thing and fixed it and would like to share with you so that you may correct me if I am wrong or it would be useful for others.
I observed that the "notify" subcommand can be run only while I am in the /var/lib/mailman/mailman directory where my mailman core is installed (through GITHUB).
And moreover I installed all the components of the mailman using the root user only and so there is no mailman user on my machine.
And hence I wrote the below entry in the crontab directly and found it is working fine.
00 08 * * * cd /var/lib/mailman/mailman; /var/lib/mailman/venv3/bin/python /var/lib/mailman/venv3/bin/mailman notify
Please correct me if my analysis is wrong or please also suggest me if I could work it out in a more optimal manner.