We've historically had our own table index introspection support, which

was needed for Django 1.6 and earlier due the limited information that

Django returned in DatabaseIntrospection.get_indexes(). Starting in

Django 1.7, they added a new get_constraints() method that returned a

lot more information, and in more recent releases (3.2+), this takes

into account information needed for expression-based constraints.
This change adds BaseEvolutionOperations.get_constraints_for_table().

It will call get_constraints() if available, falling back to

get_indexes_for_table() on Django 1.6.
This is fully compatible with our prior logic, and ensures compatibility

going forward.

Unit tests pass for all versions of Python, Django, and all database

types (across major versions of supported databases).