The old version 1 signature format was built to be somewhat compact,

with a project signature mixing version information and app signatures,

an app signature consisting solely of its models, and a field's signature

mixing the field type (which was a class reference), a possible related

model, and field attributes. The whole thing was serialized to the

pickle v0 format, which isn't the most easy-to-debug or most-portable

format out there.
The new version 2 signature format provides a bit more organization.

Now, a project keeps its app signatures inside of an "apps" entry. App

signatures keep its models under a "models" entry. Field signatures keep

its attributes under "attrs", and provide a string-based class name for

the field instead of a class reference (making it easier to fix entries

if fields move around). It's serialized to JSON, which is easier to

inspect and is far more future-proof.
The change is entirely transparent to the rest of Django Evolution, now

that we use the signature classes everywhere. Versions are only factored

in during serialization/deserialization.
Going forward, all signatures written to the database will be

JSON-encoded v2 signatures. This means that databases utilizing the

upcoming version of Django Evolution will not be compatible with older

versions of Django Evolution.

Unit tests pass on Python 2.7/Django 1.6 and Python 3.7/Django 1.11.