Provide import statement for Unique constraint

It's not clear from the docs example where the constraint lives.

docs/constraints.rst

@@ -53,6 +53,8 @@ Uniqueness constraint is declared thusly:
 
 .. code-block:: python
     
+    from datalite.constraints import Unique
+    
     @datalite("db.db")
     @dataclass
     class Student: