The previous version of Argon2 uses encoded hashes of the form:
   $argon2d$m=8,t=1,p=1$<salt>$<data>

The new version of Argon2 adds its version into the hash:
   $argon2d$v=19$m=8,t=1,p=1$<salt>$<data>

This lets Django handle both version properly.