The PIN should be in the form [0-9]+ and in order to enforce that the input function is used. By the way, in python 2.x the input function evaluates the input. Since the admin password can be retrieved as users['admin'], when we pass that exact string as PIN, python evaluates it for us, and stores the correct admin password in the password variable, allowing the login.
$ python ilovebugs.py
Hey welcome to the admin panel
Commands: REG, LOGIN
Pin ([0-9]+): users['admin']
The FLAG is what you entered in the "Pin" field to get here!