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.

Modules/_threadmodule.c

@@ -1276,10 +1276,10 @@ exception is raised, and the stack size is unmodified.  32k bytes\n\
 sufficient stack space for the interpreter itself.\n\
 \n\
 Note that some platforms may have particular restrictions on values for\n\
-the stack size, such as requiring a minimum stack size larger than 32kB or\n\
+the stack size, such as requiring a minimum stack size larger than 32 KiB or\n\
 requiring allocation in multiples of the system memory page size\n\
 - platform documentation should be referred to for more information\n\
-(4kB pages are common; using multiples of 4096 for the stack size is\n\
+(4 KiB pages are common; using multiples of 4096 for the stack size is\n\
 the suggested approach in the absence of more specific information).");
 
 static PyMethodDef thread_methods[] = {