StreamlitCallbackHandler (#6315)

A new implementation of `StreamlitCallbackHandler`. It formats Agent
thoughts into Streamlit expanders.

You can see the handler in action here:
https://langchain-mrkl.streamlit.app/

Per a discussion with Harrison, we'll be adding a
`StreamlitCallbackHandler` implementation to an upcoming
[Streamlit](https://github.com/streamlit/streamlit) release as well, and
will be updating it as we add new LLM- and LangChain-specific features
to Streamlit.

The idea with this PR is that the LangChain `StreamlitCallbackHandler`
will "auto-update" in a way that keeps it forward- (and backward-)
compatible with Streamlit. If the user has an older Streamlit version
installed, the LangChain `StreamlitCallbackHandler` will be used; if
they have a newer Streamlit version that has an updated
`StreamlitCallbackHandler`, that implementation will be used instead.

(I'm opening this as a draft to get the conversation going and make sure
we're on the same page. We're really excited to land this into
LangChain!)

#### Who can review?

@agola11, @hwchase17

tests/unit_tests/callbacks/test_streamlit_callback.py

@@ -0,0 +1,86 @@
+import builtins
+import unittest
+from typing import Any
+from unittest import mock
+from unittest.mock import MagicMock
+
+from langchain.callbacks.streamlit import StreamlitCallbackHandler
+
+
+class TestImport(unittest.TestCase):
+    """Test the StreamlitCallbackHandler 'auto-updating' API"""
+
+    def setUp(self) -> None:
+        self.builtins_import = builtins.__import__
+
+    def tearDown(self) -> None:
+        builtins.__import__ = self.builtins_import
+
+    @mock.patch("langchain.callbacks.streamlit._InternalStreamlitCallbackHandler")
+    def test_create_internal_handler(self, mock_internal_handler: Any) -> None:
+        """If we're using a Streamlit that does not expose its own
+        StreamlitCallbackHandler, use our own implementation.
+        """
+
+        def external_import_error(
+            name: str, globals: Any, locals: Any, fromlist: Any, level: int
+        ) -> Any:
+            if name == "streamlit.external.langchain":
+                raise ImportError
+            return self.builtins_import(name, globals, locals, fromlist, level)
+
+        builtins.__import__ = external_import_error  # type: ignore[assignment]
+
+        parent_container = MagicMock()
+        thought_labeler = MagicMock()
+        StreamlitCallbackHandler(
+            parent_container,
+            max_thought_containers=1,
+            expand_new_thoughts=True,
+            collapse_completed_thoughts=False,
+            thought_labeler=thought_labeler,
+        )
+
+        # Our internal handler should be created
+        mock_internal_handler.assert_called_once_with(
+            parent_container,
+            max_thought_containers=1,
+            expand_new_thoughts=True,
+            collapse_completed_thoughts=False,
+            thought_labeler=thought_labeler,
+        )
+
+    def test_create_external_handler(self) -> None:
+        """If we're using a Streamlit that *does* expose its own callback handler,
+        delegate to that implementation.
+        """
+
+        mock_streamlit_module = MagicMock()
+
+        def external_import_success(
+            name: str, globals: Any, locals: Any, fromlist: Any, level: int
+        ) -> Any:
+            if name == "streamlit.external.langchain":
+                return mock_streamlit_module
+            return self.builtins_import(name, globals, locals, fromlist, level)
+
+        builtins.__import__ = external_import_success  # type: ignore[assignment]
+
+        parent_container = MagicMock()
+        thought_labeler = MagicMock()
+        StreamlitCallbackHandler(
+            parent_container,
+            max_thought_containers=1,
+            expand_new_thoughts=True,
+            collapse_completed_thoughts=False,
+            thought_labeler=thought_labeler,
+        )
+
+        # Streamlit's handler should be created
+        mock_streamlit_module.StreamlitCallbackHandler.assert_called_once_with(
+            parent_container,
+            max_thought_containers=1,
+            expand_new_thoughts=True,
+            collapse_completed_thoughts=False,
+            thought_labeler=thought_labeler,
+        )