…not keep reference to returned object (pythonGH-1560)

* bpo-27144: concurrent.futures as_complie and map iterators do not keep
reference to returned object

* Some nits.  Improve wordings in docstrings and comments, and avoid relying on
sys.getrefcount() in tests.
(cherry picked from commit 97e1b1c)

…not keep reference to returned object (GH-1560) (#3266)

bpo-27144: concurrent.futures as_complie and map iterators do not keep
reference to returned object

(cherry picked from commit 97e1b1c)

* 'master' of https://github.com/python/cpython: (601 commits)
  remove check for bug last seem in Solaris 9 (python#3285)
  Change code owners for hashlib and ssl to the crypto team (python#3284)
  bpo-31281: Fix pathlib.Path incompatibility in fileinput (pythongh-3208)
  remove autoconf check for select() (python#3283)
  remove configure check for 'volatile' (python#3281)
  Add missing _sha3 module to Setup.dist (python#2395)
  bpo-12383: Also ignore __PYVENV_LAUNCHER__ (python#3278)
  bpo-9146: add the missing NEWS entry. (python#3275)
  Fix a c.f.as_completed() refleak previously introduced in bpo-27144 (python#3270)
  bpo-31185: Fixed miscellaneous errors in asyncio speedup module. (python#3076)
  remove a redundant lower in urllib.parse.urlsplit (python#3008)
  bpo-31323: Fix reference leak in test_ssl (python#3263)
  bpo-31250, test_asyncio: fix EventLoopTestsMixin.tearDown() (python#3264)
  bpo-31326: ProcessPoolExecutor waits for the call queue thread (python#3265)
  bpo-27144: concurrent.futures as_complete and map iterators do not keep reference to returned object (python#1560)
  bpo-31250, test_asyncio: fix dangling threads (python#3252)
  bpo-31217: Fix regrtest -R for small integer (python#3260)
  bpo-30096: Use ABC in abc reference examples (python#1220)
  bpo-30737: Update DevGuide links to new URL (pythonGH-3228)
  [Trivial] Remove now redundant assert (python#3245)
  ...

…ep reference to returned object (python#1560)

* bpo-27144: concurrent.futures as_complie and map iterators do not keep
reference to returned object

* Some nits.  Improve wordings in docstrings and comments, and avoid relying on
sys.getrefcount() in tests.

…ted()`

This was possible before.  pythonGH-1560 introduced a regression after 3.6.2 got
released where only sequences were accepted now.  This commit addresses this
problem.

…completed()` (pythonGH-3830)

This was possible before.  pythonGH-1560 introduced a regression after 3.6.2 got
released where only sequences were accepted now.  This commit addresses this
problem.
(cherry picked from commit 574562c)

…ted()` (#3830)

This was possible before.  GH-1560 introduced a regression after 3.6.2 got
released where only sequences were accepted now.  This commit addresses this
problem.

…completed()` (GH-3830) (#3831)

This was possible before.  GH-1560 introduced a regression after 3.6.2 got
released where only sequences were accepted now.  This commit addresses this
problem.
(cherry picked from commit 574562c)

…completed()` (pythonGH-3830) (python#3831)

This was possible before.  pythonGH-1560 introduced a regression after 3.6.2 got
released where only sequences were accepted now.  This commit addresses this
problem.
(cherry picked from commit 574562c)

Python issues:

+ python/cpython#1560
+ python/cpython#3270
+ python/cpython#3830

Python issues:

+ python/cpython#1560
+ python/cpython#3270
+ python/cpython#3830