Add unique constraint, None option

datalite/constraints.py

@@ -0,0 +1,26 @@
+"""
+datalite.constraints module introduces constraint
+    types that can be used to hint field variables,
+    that can be used to signal datalite decorator
+    constraints in the database.
+"""
+from typing import TypeVar, Union, Tuple
+
+T = TypeVar('T')
+
+
+class ConstraintFailedError(Exception):
+    """
+    This exception is raised when a Constraint fails.
+    """
+    pass
+
+
+"""
+Dataclass fields hinted with this type signals
+    datalite that the bound column of this
+    field in the table is NOT NULL and UNIQUE.
+"""
+Unique = Union[Tuple[T], T]
+
+