More idlelib cleanup inspired by pyflakes.
Merge with 3.4
Use try-finally idiom in example for locks in multiprocessing
Merge 3.4
Issue #5800: headers parameter of wsgiref.headers.Headers is now optional.

Patch by Pablo Torres Navarrete and SilentGhost.
Normalize style per PEP-8
Issue #21090: io.FileIO.readall() does not ignore I/O errors anymore. Before,
it ignored I/O errors if at least the first C call read() succeed.
(Merge 3.4) Issue #21090: io.FileIO.readall() does not ignore I/O errors
anymore. Before, it ignored I/O errors if at least the first C call read()
succeed.
asyncio: sync with Tulip

* _UnixSubprocessTransport: fix file mode of stdin. Open stdin in write mode,
  not in read mode
* Examples: close the event loop at exit
* More reliable CoroWrapper.__del__. If the constructor is interrupted by
  KeyboardInterrupt or the coroutine objet is destroyed lately, some the
  _source_traceback attribute doesn't exist anymore.
* repr(Task): include also the future the task is waiting for
(Merge 3.4) asyncio: sync with Tulip

* _UnixSubprocessTransport: fix file mode of stdin. Open stdin in write mode,
  not in read mode
* Examples: close the event loop at exit
* More reliable CoroWrapper.__del__. If the constructor is interrupted by
  KeyboardInterrupt or the coroutine objet is destroyed lately, some the
  _source_traceback attribute doesn't exist anymore.
* repr(Task): include also the future the task is waiting for
Fix guidance for subclassing collections.Set()
merge
Issue #21755: Skip {Frozen,Source}_DeadlockAvoidanceTests tests when
Python is built without threads.
Update docs about tp_richcompare
Merge 3.4
Issue #21151: Fixed a segfault in the winreg module.

When ``None`` was passed as a ``REG_BINARY`` value to SetValueEx,
PyMem_DEL was called on an uninitialized buffer.  Patch by John Ehresman.

(Also an incidental typo fix in a comment in test_winreg)
Closes #21151: Merge with 3.4
#15014: Add 'auth' command to implement auth mechanisms and use it in login.

Patch by Milan Oberkirch.
Add an index entry for exec() builtin.

Reported by Draic Kin on docs@.
Merge from 3.4.