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/Modules/_io/winconsoleio.c b/Modules/_io/winconsoleio.c
index 6600fde..30d1c76 100644
--- a/Modules/_io/winconsoleio.c
+++ b/Modules/_io/winconsoleio.c
@@ -31,7 +31,7 @@
 #if BUFSIZ < (16*1024)
 #define SMALLCHUNK (2*1024)
 #elif (BUFSIZ >= (2 << 25))
-#error "unreasonable BUFSIZ > 64MB defined"
+#error "unreasonable BUFSIZ > 64 MiB defined"
 #else
 #define SMALLCHUNK BUFSIZ
 #endif