fix: process_coupon_assignment_sheet warnings and errors

[Anas12091101]

What are the relevant tickets?

https://github.com/mitodl/hq/issues/3872

Description (What does it do?)

The --help in process_coupon_assignment_sheet.py was throwing error due to the addition of comma(,) in the --force argument help description making it a tuple. This PR fixes this issue and also fixes the warnings when running the command

How can this be tested?

• Validate that no error comes when running python manage.py process_coupon_assignment_sheet --help command
• Add the DRIVE_SERVICE_ACCOUNT_CREDS and append the service account email address in SHEETS_ADMIN_EMAILS in mitxpro/settings.py
• Create a google spreadsheet and run the command python manage.py process_coupon_assignment_sheet --id <sheet-id>
• Validate that there's no warning when running the command

[asadali145]

@Anas12091101 Is this ready for review?