Py_buffer::format is implemented as `char*` instead of `const char*`

**Bug report**

The format member in `Py_buffer` is [documented](https://docs.python.org/3.10/c-api/buffer.html#c.Py_buffer.format) to have the type `const char*`. However the actual type is `char*`:
https://github.com/python/cpython/blob/ec091bd47e2f968b0d1631b9a8104283a7beeb1b/Include/pybuffer.h#L28

