Issue #9815: assertRaises now tries to clear references to local variables in the exception's traceback.
Issue #9307: document the various Py_TPFLAGS_*_SUBCLASS flags.  Patch by Yury V. Zaytsev.
Issue #21026: Augment site doc based on experiments. Patch by Carol Willing.
Closes 21048: Index 'as' in import and with statements.
Issue #21055: Index (augmented) assignment symbols.