bpo-29942: Fix the use of recursion in itertools.chain.from_iterable. (#911)

* bpo-29942: Fix the use of recursion in itertools.chain.from_iterable.

Fix the use of recursion in itertools.chain.from_iterable. Using recursion
is unnecessary, and can easily cause stack overflows, especially when
building in low optimization modes or with Py_DEBUG enabled.
(cherry picked from commit 5466d4af5fe76ec0a5fbc8a05675287d9e8e9d14)
3 files changed