bpo-37972: unittest.mock._Call now passes on __getitem__ to the __getattr__ chaining so that call() can be subscriptable (GH-15565)

* bpo-37972: unittest.mock._Call now passes on __getitem__ to the __getattr__ chaining so that call() can be subscriptable

* 📜🤖 Added by blurb_it.

* Update 2019-08-28-21-40-12.bpo-37972.kP-n4L.rst

added name of the contributor

* bpo-37972: made all dunder methods chainable for _Call

* bpo-37972: delegate only attributes of tuple instead to __getattr__