Update __init__.py (#9955)

Add SQLDatabaseSequentialChain Class to __init__.py so it can be
accessed and used

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- Description: SQLDatabaseSequentialChain is not found when importing
Langchain_experimental package, when I open __init__.py
Langchain_expermental.sql, I found that SQLDatabaseSequentialChain is
imported and add to __all__ list
- Issue: SQLDatabaseSequentialChain is not found in
Langchain_experimental package
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libs/experimental/langchain_experimental/sql/__init__.py

@@ -1,4 +1,4 @@
 """Chain for interacting with SQL Database."""
-from langchain_experimental.sql.base import SQLDatabaseChain
+from langchain_experimental.sql.base import SQLDatabaseChain, SQLDatabaseSequentialChain
 
-__all__ = ["SQLDatabaseChain"]
+__all__ = ["SQLDatabaseChain", "SQLDatabaseSequentialChain"]