Serialized signature data can reference field class paths that aren't

valid across all versions of Django. Depending on when the signatures

were created, old class paths could be used that won't work on newer

Django versions, and need to be updated.
This change introduces a special unpickler that can remap class paths

for fields, allowing these signatures to be deserialized across all

versions of Django. It's only used if needed, favoring the C-optimized

unpickler instead.

Unit tests pass on all supported versions of Django.
Tested with locally-generated signatures that exhibited the problem.