Missing `NonLocalVariableRead` from dictionary access

[lukarade]

Describe the bug

The NonLocalVariableRead caused by accessing the instance variable self._dict in the following function was not detected.

# matplotlib.spines.__getattr__.551.4
def __getattr__(self, name):
    try:
        return self._dict[name] # Impure NonLocalVariableRead
    except KeyError:
        raise AttributeError(f"'Spines' object does not contain a '{name}' spine")

To Reproduce

1. Run purity analysis on the matplotlib library
2. Check the result file for the function: matplotlib.spines.getattr.551.4

Expected behavior

This should be detected even though it is access of the dictionary.

Screenshots (optional)

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Additional Context (optional)

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