Upgrade dmutils to 8.1.1
Upgrade dmutils to 8.1.1

This includes:
- alphagov/digitalmarketplace-utils#170
- alphagov/digitalmarketplace-utils#167
- alphagov/digitalmarketplace-utils#169

alphagov/digitalmarketplace-api,alphagov/digitalmarketplace-api,alphagov/digitalmarketplace-api