[clang] [libclang/python] Add typing annotations for the Cursor class (PR #138103)

[Vlad Serebrennikov via cfe-commits]

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@@ -77,6 +77,8 @@ Clang Frontend Potentially Breaking Changes
 
 Clang Python Bindings Potentially Breaking Changes
 --------------------------------------------------
+- Calling methods on null-cursors now leads to an exception.
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Endilll wrote:

```suggestion
- Almost all `Cursor` methods now assert that they are called on non-null cursors. Most of the time null cursors were mapped to `None`, so no widespread breakages are expected.
```

https://github.com/llvm/llvm-project/pull/138103