asyncio: Fix pyflakes warnings: remove unused variables and imports
asyncio: simplify test_events.py, don't use non local variables and don't call
assert methods in coroutines.

It also simplify merges from Tulip to Trollius (Python 2 does not support non
local variables).
Issue #20759: Fix some typos in the mock docs.
Close #20759: Fix some typos in the mock docs. (Merge with 3.3)
asyncio/windows_events.py: use more revelant names to overlapped callbacks

For example: "finish_recv", not just "finish".
Fix several C-API doc typos caught by tomo cocoa on docs@.

The signature and description of PyException_SetCause now use "cause"
rather than "ctx" to match the code.
Merge with 3.3
Added tests for issue #20501.
Added tests for issue #20501.