Replace KB unit with KiB (#4293)

kB (*kilo* byte) unit means 1000 bytes, whereas KiB ("kibibyte")
means 1024 bytes. KB was misused: replace kB or KB with KiB when
appropriate.

Same change for MB and GB which become MiB and GiB.

Change the output of Tools/iobench/iobench.py.

Round also the size of the documentation from 5.5 MB to 5 MiB.
diff --git a/Doc/library/multiprocessing.rst b/Doc/library/multiprocessing.rst
index 80a87b8..3619ccc 100644
--- a/Doc/library/multiprocessing.rst
+++ b/Doc/library/multiprocessing.rst
@@ -1034,7 +1034,7 @@
       Send an object to the other end of the connection which should be read
       using :meth:`recv`.
 
-      The object must be picklable.  Very large pickles (approximately 32 MB+,
+      The object must be picklable.  Very large pickles (approximately 32 MiB+,
       though it depends on the OS) may raise a :exc:`ValueError` exception.
 
    .. method:: recv()
@@ -1071,7 +1071,7 @@
 
       If *offset* is given then data is read from that position in *buffer*.  If
       *size* is given then that many bytes will be read from buffer.  Very large
-      buffers (approximately 32 MB+, though it depends on the OS) may raise a
+      buffers (approximately 32 MiB+, though it depends on the OS) may raise a
       :exc:`ValueError` exception
 
    .. method:: recv_bytes([maxlength])