Fix formatting warnings for PyBytesWriter_Format

[amolenaar]

Add C formatting warnings and fix declaration.

With extra formatting warnings, as we have in PyGObject, we get warnings like:

In file included from ../../gi/pygi-hashtable.c:21:
../../subprojects/pythoncapi-compat/pythoncapi_compat.h:2537:41: warning: format string is not a string literal [-Wformat-nonliteral]
 2537 |     PyObject *str = PyBytes_FromFormatV(format, vargs);
      |                                         ^~~~~~

This PR adds formatting warnings (errors) to the test build and addresses the format-nonliteral warning by providing an appropriate prototype for this function.

[vstinner]

I cannot reproduce the issue with GCC 15.2.1. What's your GCC version?

My test: run python3 runtests.py --current -v with setup.py changes.

[amolenaar]

I was testing on macOS (15) with Python 3.13.7. I suppose Clang is a bit more picky (?).

I did a (test) CI run with only the extra warnings added to setup.py, which fails .

[vstinner]

I was testing on macOS (15) with Python 3.13.7. I suppose Clang is a bit more picky (?).

Ah you're using clang, ok. Using clang, I can reproduce the compiler warning.

[vstinner]

Merged, thanks for the fix.

[vstinner]

I'm surprised that static inline int PyUnicodeWriter_Format(PyUnicodeWriter *writer, const char *format, ...) doesn't emit a similar warning.