Large backups fail with gio+google-drive backend

[Andrew C. Dingman]

This was reported nearly a year ago as a Deja Dup bug, but it's really a problem with Duplicity itself. https://bugs.launchpad.net/deja-dup/+bug/1747646

Essentially, small backups work, but large ones eventually fail with the following message:

Giving up after 5 attempts. Error: Authentication required: {
 "error": {
  "errors": [
   {
    "domain": "global",
    "reason": "authError",
    "message": "Invalid Credentials",
    "locationType": "header",
    "location": "Authorization"
   }
  ],
  "code": 401,
  "message": "Invalid Credentials"
 }
}

Resuming with the same unmodified command appears to work.