gh-141045: Document that shutil.Error is a subclass of OSError

[Lakshya-Upadhyaya]

Fixes #141045

This updates the documentation for shutil.Error to note that it is a subclass of OSError. This clarifies that code catching OSError will also catch shutil.Error, similar to how http.cookiejar.LoadError is documented.

File modified:

• Doc/library/shutil.rst

• Issue: Document the fact that shutil.Error is a subclass of OSError #141045

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