Comment 4 for bug 1585913

I've tried to implement a solution for this, based on the work done by tsimonq2.

The implementation still autodetects the lenght of the digest, but now is specific about the algorithm. For example:
source-checksum: sha2/035ae7da4bd0ff39960466353e0810f51d17193a13e8b75e767391820aed484c
source-checksum: sha1/30fdfacb19b557a762932c5a3a867cdc698e447f

SHA3 support is commented out, as is not yet implented in python3.5 hashlib; it will be in python 3.6: https://docs.python.org/3.6/whatsnew/3.6.html#hashlib