[3.12] gh-129573: Fix abort from non-string suggestions in calculate_suggestions

[devdanzin]

This PR adds a simple check that suggestion candidates are strings in calculate_suggestions, avoiding an abort in debug builds from code like below:

class Parent:
    def __dir__(self):
        return [0]

class WithNonStringAttrs(Parent):
    blech = None

WithNonStringAttrs().bluch

Found using fusil by @vstinner.

• Issue: Failed assertion in _PyUnicode_Equal from calculate_suggestions with non-string candidate #129573

[ZeroIntensity]

Thanks for doing this! See my comment: #129573 (comment)

The offending code isn't a problem on 3.13+, but it still exists. Please open a new PR for the main branch.