The C tokenizer's common.h redefines malloc/realloc/free to the
PyObject_* family but left calloc pointing at libc. Entity parsing in
Tokenizer_really_parse_entity allocates with calloc and later releases
the buffer with free (= PyObject_Free), which is undefined behavior:
under PYTHONMALLOC=debug it aborts in _PyMem_DebugRawFree, and
otherwise it can silently corrupt the heap.

Route calloc through PyObject_Calloc alongside the other allocator
macros so both calloc sites match the surrounding free.

Fixes #352.