Add padding in list allocation

[serhiy-storchaka]

Since the Python memory allocator has granularity of 16 bytes, allocating 24 bytes does not save memory in comparison with allocating 32 bytes. One of changes in #82554 was to round the allocated size for list up to 4 items (16 bytes on 32-bit platform and 32 bytes of 64-bit platform). #87740 reverted this. Therefore, if you create an empty list, then extend it with 1 item two times, you will need two allocations, even if the first allocation has a space for yet one item (but list does not know about this).

>>> import sys
>>> a = []
>>> sys.getsizeof(a)
56
>>> a += [1]
>>> sys.getsizeof(a)
64
>>> a += [2]
>>> sys.getsizeof(a)
120