#21225: copy docstrings from base classes
Closes #21026: Augment site doc based on experiments. Patch by Carol Willing.
Closes 21048: Index 'as' in import and with statements.
Closes #21055: Index (augmented) assignment symbols.
Issue #21321: itertools.islice() now releases the reference to the source iterator when the slice is exhausted.

Patch by Anton Afanasyev.
Issue #19630 Emphasise that the file parameter to marshal.dump must be a real file object
Issue #18604: Consolidated checks for GUI availability.

test_support._is_gui_available is now defined the same way on every
platform, and now includes the Windows-specific check that had been in the
Windows version of _is_gui_available and the OSX-specific check that was
in runtktests.check_tk_availability.  Also, every platform checks whether
Tk can be instantiated (if the platform-specific checks passed).