Bug 1653190: Fix handling of checksum format in python 2 vs 3; r=mtabara a=tomprince

We switched to using python 3 to run generate-checksums, this caused the
checksum files to have an extra b'' in them. The original fix caused the
generation of merkletree to fail. This moves the code that encodes the hash
closer to where it is output.

Differential Revision: https://phabricator.services.mozilla.com/D85196