There are field attributes normally relevant to a column's definition

that some fields accept that don't have any actual impact for that

particular field. An example is ManyToManyField(null=True), which has

historically been allowed but has since resulted in warnings.
Django Evolution has been happy to try to apply these changes to the

database, but that just results in bad SQL and a failed evolution.
This change adds support for defining ignorable attributes for different

field types. This currently only consists of null for

ManyToManyField, but may be useful for more fields in the future.
The signature still retains these attributes, and they're factored in

for diffs and evolution hints. They're only ignored at the SQL

generation level.

Unit tests pass for all databases.