Django-mssql-backend

* Exclude indexes when deleting unique constraints (51)
* Fix issue 45 correctly reinstate nullable unique constraints (47)
* Fix issue 38 DROP index vs constraint issue (39)

* Add regex support (43)

* Fix Exists (29, 30)
* Add support for SQL Server 2019 (32)

* Add Django 3 support (19)
* Update operations.py (27)

- Add support for nullable fields in unique_together 24

- Revert native UUID changes (8bf0154)