Use a small stack allocated in the C stack for up to 5 iterator functions,
otherwise allocates a stack on the heap memory.

Patch by Ivan Levkivskyi, docs by Neil Girdhar.

In asyncio.locks.Lock.acquire(): Avoid deadlock when a cancelled future is in self._waiters. (Merge 3.5->3.6)

The @reap_threads decorator made the test wait (for up to 1 s) until
background threads have finished.  Calling join() with a timeout should be
equivalent.

Issue #27809: PyEval_CallObjectWithKeywords() doesn't increment temporary the
reference counter of the args tuple (positional arguments). The caller already
holds a strong reference to it.

Issue #27809: Helper function optimized to create an exception: use fastcall
whenever possible.

Modify:

* init_subclass()
* builtin___build_class__()

Fix also a bug in init_subclass(): check for super() failure.

Issue #27809. _PyObject_FastCallDict() now supports keyword arguments, and so
the args==NULL fast-path can also be used when kwargs is not NULL.

Modify:

* builtin_sorted()
* classmethoddescr_call()
* methoddescr_call()
* wrapperdescr_call()

* Rename nm to name
* PEP 7: add { ... } to if/else blocks

Issue #27809:

* Rename _PyFunction_FastCall() to _PyFunction_FastCallDict()
* Rename _PyCFunction_FastCall() to _PyCFunction_FastCallDict()
*  _PyFunction_FastCallDict() now supports keyword arguments

Issue #27809:

* Rename _PyObject_FastCall() function to _PyObject_FastCallDict()
* Add _PyObject_FastCall(), _PyObject_CallNoArg() and _PyObject_CallArg1()
  macros calling _PyObject_FastCallDict()

This was found by PVS-Studio:

V595 The 'def' pointer was utilized before it was verified
against nullptr. Check lines: 286, 292. pystate.c 286

Initial patch by Christian Heimes.

Use flush=True to try to get a warning which is missing in buildbots.

Use also f-string to make the code shorter.

* main.py: remove unused import
* runtest: simplify runtest_inner() parameters, reuse ns parameter

Issue #27128. When a Python function is called with no arguments, but all
parameters have a default value: use default values as arguments for the fast
path.

Issue #27539: Fix unnormalised Fraction.__pow__ result for negative exponent and base. Thanks Vedran Čačić.