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/test/test_threading.py b/Lib/test/test_threading.py
index a4887af..007581d 100644
--- a/Lib/test/test_threading.py
+++ b/Lib/test/test_threading.py
@@ -132,10 +132,10 @@
         # Kill the "immortal" _DummyThread
         del threading._active[ident[0]]
 
-    # run with a small(ish) thread stack size (256kB)
+    # run with a small(ish) thread stack size (256 KiB)
     def test_various_ops_small_stack(self):
         if verbose:
-            print('with 256kB thread stack size...')
+            print('with 256 KiB thread stack size...')
         try:
             threading.stack_size(262144)
         except _thread.error:
@@ -144,10 +144,10 @@
         self.test_various_ops()
         threading.stack_size(0)
 
-    # run with a large thread stack size (1MB)
+    # run with a large thread stack size (1 MiB)
     def test_various_ops_large_stack(self):
         if verbose:
-            print('with 1MB thread stack size...')
+            print('with 1 MiB thread stack size...')
         try:
             threading.stack_size(0x100000)
         except _thread.error: