Error:

google.auth.exceptions.RefreshError: ('invalid_grant: Token has been expired or revoked.', {'error': 'invalid_grant', 'error_description': 'Token has been expired or revoked.'})

However, another app I use, with a different account, never runs into any issues. I use the same Python OAuth Quickstart for both.
Token has been expired or revoked.

Basically means just that either the user has revoked your access or google has.  Users can remove your access directly in their google account when ever they want to.
Google expired tokens

If you are using a gmail scope, and the user changes their password. Your refresh token will probably be revoked.
If your app is still in testing and the refresh token is more then seven days old the users consent will be removed and the refresh tokens will be revoked.
If the refresh token has not been used in more then six months the refresh token will be revoked.
If the user authroizes you app you get a refresh token, if the do it again you get another refresh token. both will work.  You can have up to 50 outstanding refresh tokens for a user.  If you request again then the first one will be expired.  Ensure you are always storing the most recent refresh token.

no matter what the cause your application should be configured in a way as to request authorization from them again if the refresh token has expired.
I've been working with the Google API. Sometimes my refresh token refreshes and other times it fails and causes a 'RefreshError.' Why? How to fix?