Fix test_ne in test_cmp.py for Python 3.13

[frenzymadness]

Summary

Compiler in Python 3.13+ strips indents from docstrings so they need to be compared without it for new Pythons.

Tested with 3.13.0a4

Fixes: https://github.com/python-attrs/attrs/issues/1228

Pull Request Check List

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• [ ] Added tests for changed code. Our CI fails if coverage is not 100%.
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[frenzymadness]

I found the attr._compat module so I use it now.