Run mypy on CI, fix or ignore typing issues (jupyterlab#987)

* Run mypy on CI

* Rename, add mypy to test deps

* Fix typing jupyter-ai codebase (mostly)

* Three more cases

* update deepmerge version specifier

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Co-authored-by: David L. Qiu <david@qiu.dev>

packages/jupyter-ai/jupyter_ai/handlers.py

@@ -45,8 +45,8 @@
     from jupyter_ai_magics.embedding_providers import BaseEmbeddingsProvider
     from jupyter_ai_magics.providers import BaseProvider
 
-    from .context_providers import BaseContextProvider
     from .history import BoundedChatHistory
+    from .context_providers import BaseContextProvider
 
 
 class ChatHistoryHandler(BaseAPIHandler):