gh-38391: get rid of one sage_eval in complex numbers
    
this is parsing in a simple way the string input for complex number
constructor, avoiding usage of `sage_eval` there


### 📝 Checklist

- [x] The title is concise and informative.
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preview.
    
URL: #38391
Reported by: Frédéric Chapoton
Reviewer(s): Matthias Köppe

Author: Release Manager