bpo-40052: Fix alignment issue in PyVectorcall_Function() (GH-23999)

bpo-40052: Fix alignment issue in PyVectorcall_Function() (GH-23999)

```
In file included from /usr/include/python3.8/Python.h:147:
In file included from /usr/include/python3.8/abstract.h:837:
/usr/include/python3.8/cpython/abstract.h:91:11: error: cast from 'char *' to 'vectorcallfunc *'
(aka 'struct _object *(**)(struct _object *, struct _object *const *, unsigned long, struct _object *)')
increases required alignment from 1 to 8 [-Werror,-Wcast-align]

    ptr = (vectorcallfunc*)(((char *)callable) + offset);
          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.
```
Co-Authored-By: Andreas Schneider <asn@cryptomilk.org>
Co-Authored-By: Antoine Pitrou <antoine@python.org>