Merge .pyi type stubs inline (#563)

* Merged types into .py code.

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* Fix: nox -rs generate.

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* Updated CHANGELOG.

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* Use lowest common python version for lint.

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* Fix: don't typeshed.

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* Removed unneeded comment.

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* Simplify OPENSEARCH_URL.

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* Fix: positional ignore_status used as chunk_size.

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* Fix: parse version string.

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* Remove future annotations for Python 3.6.

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* Fix: types in documentation.

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* Improve CHANGELOG text.

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* Re-added missing separator.

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* Remove duplicate licenses.

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* Get rid of Optional[Any].

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* Fix docs with AsyncOpenSearch.

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* Fix: undo comment.

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DEVELOPER_GUIDE.md

@@ -66,9 +66,11 @@ If you want to auto-start one, the following will start a new instance and run t
 If your OpenSearch docker instance is running, you can execute the test suite directly.
 
 ```
-$ nox -rs test
+$ nox -rs test-3.9
 ```
 
+Substitute `3.9` with your Python version above, or use `nox -rs test` to run with multiple.
+
 To run tests against different versions of OpenSearch, use `run-tests [with/without security] [version]`:
 
 ```