Fix #663 - Fixed incompatibility of DatabaseSchemaIntrospection with django

[adrienrusso]

This PR fixes issue #663.

Instead of duplicating the code of the DatabaseSchemaIntrospection methods, the schema search path can simply be set on the cursor.

This class was clearly out of date and some of its functions no longer exist in django.

[tomturner]

@adrienrusso Thanks I will have a look at some point

[adrienrusso]

@tomturner any update?

[tomturner]

@adrienrusso thank you for doing this. Sorry for the wait it took a bit a time for me to test hence the deley.

Notes to myself on how I tested it

    from django.db import connection
    from django.apps import apps
    tables = connection.introspection.table_names()
    seen_models = connection.introspection.installed_models(tables)
    for model in apps.get_models():
        print(model)

[adrienrusso]

Thanks for the review and the merge 👌