Patch by Jaysinh Shukla and Stéphane Wirtel.

When the body object is a file, its size is no longer determined with
fstat(), since that can report the wrong result (e.g. reading from a pipe).
Instead, determine the size using seek(), or fall back to chunked encoding
for unseekable files.

Also, change the logic for detecting text files to check for TextIOBase
inheritance, rather than inspecting the “mode” attribute, which may not
exist (e.g. BytesIO and StringIO).  The Content-Length for text files is no
longer determined ahead of time, because the original logic could have been
wrong depending on the codec and newline translation settings.

Patch by Demian Brecht and Rolf Krahl, with a few tweaks by me.

Although nth roots of negative numbers are real for odd n, the
statistics module doesn't make use of this. Remove support for
negative roots from the private _nth_root function, which
simplifies the test suite.

Patch by Christian Heimes, reviewed by Martin Panter.

Patch by SilengGhost.

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

Issue #27809:

* PyObject_CallMethodObjArgs(), _PyObject_CallMethodIdObjArgs() and
  PyObject_CallFunctionObjArgs() now use fast call to avoid the creation of a
  temporary tuple
* Rename objargs_mktuple() to objargs_mkstack()
* objargs_mkstack() now stores objects in a C array using borrowed references,
  instead of storing arguments into a tuple

objargs_mkstack() uses a small buffer allocated on the C stack for 5 arguments
or less, or allocates a buffer in the heap memory.

Note: this change is different than the change 0e4f26083bbb, I fixed the test
to decide if the small stack can be used or not. sizeof(PyObject**) was also
replaced with sizeof(stack[0]) since the sizeof() was wrong (but gave the same
result).

Issue #27809:

* PyObject_CallMethodObjArgs(), _PyObject_CallMethodIdObjArgs() and
  PyObject_CallFunctionObjArgs() now use fast call to avoid the creation of a
  temporary tuple
* Rename objargs_mktuple() to objargs_mkstack()
* objargs_mkstack() now stores objects in a C array using borrowed references,
  instead of storing arguments into a tuple

objargs_mkstack() uses a small buffer allocated on the C stack for 5 arguments
or less, or allocates a buffer in the heap memory.

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.