add-license-headers is changing special characters

The hook is incorrectly writing special characters with fileinput when using python 3.9

https://github.com/ansys/pyadditive/blob/main/src/ansys/additive/core/microstructure.py


Start with the add-license-headers hook and reproduce the character-changing behavior using the linked src/ansys/additive/core/microstructure.py file under Python 3.9. Compare its output before and after the hook runs; done means special characters are preserved, with regression coverage for the reported case.

Written by the indexing model from the issue text.