Add the missing "null" datatype for OpenAPI v3.0.3

Author: hathawsh

openapi_pydantic/v3/v3_0_3/datatype.py

@@ -4,6 +4,7 @@
 class DataType(str, enum.Enum):
     """Data type of an object."""
 
+    NULL = "null"
     STRING = "string"
     NUMBER = "number"
     INTEGER = "integer"