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/Lib/xmlrpc/client.py b/Lib/xmlrpc/client.py
index bd3278e..2852503 100644
--- a/Lib/xmlrpc/client.py
+++ b/Lib/xmlrpc/client.py
@@ -1046,7 +1046,7 @@
 # in the HTTP header, as described in RFC 1952
 #
 # @param data The encoded data
-# @keyparam max_decode Maximum bytes to decode (20MB default), use negative
+# @keyparam max_decode Maximum bytes to decode (20 MiB default), use negative
 #    values for unlimited decoding
 # @return the unencoded data
 # @raises ValueError if data is not correctly coded.