Serhiy Storchaka:
"""
Objects/unicodeobject.c: In function ‘_PyUnicode_FastFill’:
Objects/unicodeobject.c:10126:24: warning: passing argument 2 of ‘unicode_fill’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
"""
Yeah, I was aware of the warning. I had a local branch but I started to make many unrelated changes and so I lost track of the issue :-)
The warning should now be fixed. Thanks for the report.
Note: it was a real bug which exists since at least Python 3.6 ;-)