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/gzip.py b/Lib/gzip.py
index 76ab497..ddc7bda 100644
--- a/Lib/gzip.py
+++ b/Lib/gzip.py
@@ -308,7 +308,7 @@
             if self.mode == WRITE:
                 fileobj.write(self.compress.flush())
                 write32u(fileobj, self.crc)
-                # self.size may exceed 2GB, or even 4GB
+                # self.size may exceed 2 GiB, or even 4 GiB
                 write32u(fileobj, self.size & 0xffffffff)
             elif self.mode == READ:
                 self._buffer.close()