Datalite

* Fixed an issue in import paths.

* Add datalite.constraint module, that introduces SQLite constraints to datalite classes.
* Add Unique[T] type alias, when added a dataclass field is hinted with it, the corresponding column in the database will be NOT NULL and UNIQUE.
* Fix a previously unkown bug that made it impossible for datalite objects to be insantiated with None as an argument.

* Add datalite.constraint module, that introduces SQLite constraints to datalite classes.
* Add Unique[T] type alias, when added a dataclass field is hinted with it, the corresponding column in the database will be NOT NULL and UNIQUE.
* Fix a previously unkown bug that made it impossible for datalite objects to be insantiated with None as an argument.

* A bugfix in `datalite.migrations.basic_migrate` that caused the software to raise an exception when a column was deleted.

* Very minor bugfix

* Migration support for default values that are not `None`