> May be we should add a new function (_PyMem_RawMallocItems?) that does the same checks as PyMem_RawCalloc, but doesn't zero-initialize memory?

Please don't add new functions to the Python memory allocators. We already have too many of them :-(
https://docs.python.org/dev/c-api/memory.html

PyZlib_Malloc, PyLzma_Malloc and BZ2_Malloc exists because they use different types: 2 unsigned int (zlib), 2 size_t (lzma), 2 int (bz2). PyMem_RawMalloc() expects a single size_t.

IMHO it's fine to have a function of 5 lines of code in each module, since each module uses a different C type.