added Wikipedia retriever (#4302)

- added `Wikipedia` retriever. It is effectively a wrapper for
`WikipediaAPIWrapper`. It wrapps load() into get_relevant_documents()
- sorted `__all__` in the `retrievers/__init__`
- added integration tests for the WikipediaRetriever
- added an example (as Jupyter notebook) for the WikipediaRetriever

tests/integration_tests/retrievers/test_wikipedia.py

@@ -0,0 +1,53 @@
+"""Integration test for Wikipedia API Wrapper."""
+from typing import List
+
+import pytest
+
+from langchain.retrievers import WikipediaRetriever
+from langchain.schema import Document
+
+
+@pytest.fixture
+def retriever() -> WikipediaRetriever:
+    return WikipediaRetriever()
+
+
+def assert_docs(docs: List[Document], all_meta: bool = False) -> None:
+    for doc in docs:
+        assert doc.page_content
+        assert doc.metadata
+        main_meta = {"title", "summary"}
+        assert set(doc.metadata).issuperset(main_meta)
+        if all_meta:
+            assert len(set(doc.metadata)) > len(main_meta)
+        else:
+            assert len(set(doc.metadata)) == len(main_meta)
+
+
+def test_load_success(retriever: WikipediaRetriever) -> None:
+    docs = retriever.get_relevant_documents("HUNTER X HUNTER")
+    assert len(docs) > 1
+    assert_docs(docs, all_meta=False)
+
+
+def test_load_success_all_meta(retriever: WikipediaRetriever) -> None:
+    retriever.load_all_available_meta = True
+    docs = retriever.get_relevant_documents("HUNTER X HUNTER")
+    assert len(docs) > 1
+    assert_docs(docs, all_meta=True)
+
+
+def test_load_success_init_args() -> None:
+    retriever = WikipediaRetriever(
+        lang="en", top_k_results=1, load_all_available_meta=True
+    )
+    docs = retriever.get_relevant_documents("HUNTER X HUNTER")
+    assert len(docs) == 1
+    assert_docs(docs, all_meta=True)
+
+
+def test_load_no_result(retriever: WikipediaRetriever) -> None:
+    docs = retriever.get_relevant_documents(
+        "NORESULTCALL_NORESULTCALL_NORESULTCALL_NORESULTCALL_NORESULTCALL_NORESULTCALL"
+    )
+    assert not docs