Merge branch 'f-type-hints' into 'dev'

Add type hints to models.py

See merge request !67

src/linkahead/common/datatype.py

@@ -24,6 +24,10 @@
 #
 
 import re
+import sys
+
+if sys.version_info >= (3, 8):
+    from typing import Literal
 
 from ..exceptions import EmptyUniqueQueryError, QueryNotUniqueError
 
@@ -34,6 +38,8 @@ DATETIME = "DATETIME"
 INTEGER = "INTEGER"
 FILE = "FILE"
 BOOLEAN = "BOOLEAN"
+if sys.version_info >= (3, 8):
+    DATATYPE = Literal["DOUBLE", "REFERENCE", "TEXT", "DATETIME", "INTEGER", "FILE", "BOOLEAN"]
 
 
 def LIST(datatype):