Assertion `(char *) aligned_storage + holder_size <= (char *)base_storage + base_allocation' failed.

Hi,

Many thanks for the great library!

We just migrated to boost 1.78 and start getting:
```
python3: libs/python/src/object/class.cpp:762: static void* boost::python::instance_holder::allocate(PyObject*, std::size_t, std::size_t, std::size_t): Assertion `(char *) aligned_storage + holder_size <= (char *)base_storage + base_allocation' failed.
```
on arm platform. On x64 everything works perfect.

I took a look into boost::python::instance_holder::allocate with debugger and see:
```
sizeof(alignment_marker_t) == 4
alignment == 4
holder_size == 16
```
so assertion is correct.

I'm a bit confused, why we need -1 at https://github.com/boostorg/python/blame/develop/src/object/class.cpp#L750?

Looking forward for your answer!