Add line explaining the "%sort" test.
(grafted from 1ea833ecaf5a9d43a886e9e73b4e2551d0d5b548)
Emphasize that people should not override __import__.

With importlib getting used more and more, changing __import__ will
not work as well as people used to hope as it will potentially bypass
importers, etc. It also will not work with importlib.import_module()
as it uses "importlib.__import__" (i.e. importlib's implementation of
import) directly and not builtins.__import__.
Issue #18755: Allow imp.load_*() loaders to have get_data() called
multiple times.
NEW entry for issue #18755
Close #17702: On error, os.environb now removes suppress the except context
when raising a new KeyError with the original key.
#18798: fix typo in test_fcntl.  Patch by Vajrasky Kok.
Issue #18757: Improved cross-references in the concurrent package.