sagelib: Remove unnecessary import of typing_extensions

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see also #34831

CC:  @EnderWannabe @fchapoton @jhpalmieri

Component: **refactoring**

Author: **Matthias Koeppe**

Branch/Commit: **[`39bd748`](https://github.com/sagemath/sagetrac-mirror/commit/39bd748020fb651e5aeb7640ffe182e493353c79)**

Reviewer: **John Palmieri**

_Issue created by migration from https://trac.sagemath.org/ticket/34859_

