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MINOR: [Python] Remove extra import from PyCapsule Interface Doc (#43399)

### Rationale for this change

The PyCapsule Interface doc includes the line

```py
from typing_extensions import Self
```

but `Self` is not used anywhere in the typing declarations, so that line can be removed.

### What changes are included in this PR?

### Are these changes tested?

### Are there any user-facing changes?

No

Authored-by: Kyle Barron <kylebarron2@gmail.com>
Signed-off-by: Dane Pitkin <dpitkin@apache.org>