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.

Lib/test/test_socket.py

@@ -5299,7 +5299,7 @@ class SendfileUsingSendTest(ThreadedTCPSocketTest):
     Test the send() implementation of socket.sendfile().
     """
 
-    FILESIZE = (10 * 1024 * 1024)  # 10MB
+    FILESIZE = (10 * 1024 * 1024)  # 10 MiB
     BUFSIZE = 8192
     FILEDATA = b""
     TIMEOUT = 2