bpo-30197: Enhance functions swap_attr() and swap_item() in test.support. (#1341)

* bpo-30197: Enhance functions swap_attr() and swap_item() in test.support.

They now work when delete replaced attribute or item inside the with
statement.  The old value of the attribute or item (or None if it doesn't
exist) now will be assigned to the target of the "as" clause, if there is
one.

* Update docstrings.