- 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
The forward ref annotations don't get updated if we only iimport with
type checking
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Co-authored-by: Abhinav Verma <abhinav_win12@yahoo.co.in>