refactor: add the ConstantKeyword field to the top-level package (#1843)

(cherry picked from commit 412708e)

Author: alirezaazadi

elasticsearch_dsl/__init__.py

@@ -42,6 +42,7 @@
     Boolean,
     Byte,
     Completion,
+    ConstantKeyword,
     CustomField,
     Date,
     DateRange,
@@ -113,6 +114,7 @@
     "Boolean",
     "Byte",
     "Completion",
+    "ConstantKeyword",
     "CustomField",
     "Date",
     "DateHistogramFacet",