Use-after-free in object::memoryview for the format description

[aguinet]

Issue description

object:memoryview pass a temporary buffer for the format to PyMemoryView_FromBuffer. By reading the source code of this function, it does not "strdup" this format string, so it supposes it's always valid.

It is not really clear in the documentation how ownership works for the format. I figured this out because I had broken format issues with memory views in https://github.com/aguinet/dragonffi/ . A temporary fix by using strdup here https://github.com/aguinet/dragonffi/blob/master/third-party/pybind11/pytypes.h#L1255 fix the problem, but leaks memory.

Reproducible example code

Hard to reproduce, as it's an use-after-free problem. Basically, creates a memoryview with a given format, and in some cases the format within the Python object will be wrong!

[YannickJadoul]

I think this was fixed by #2223? If not, please reopen!