Bump dm-utils to 6.7.1
Bump dm-utils to 6.7.1

Brings in the fix for a unicode error from the
process that generates a fingerprint for the
static asset files:

https://github.com/alphagov/digitalmarketplace-utils/pull/155

alphagov/digitalmarketplace-supplier-frontend,alphagov/digitalmarketplace-supplier-frontend,alphagov/digitalmarketplace-supplier-frontend,alphagov/digitalmarketplace-supplier-frontend