bpo-40170: Convert PyIter_Check macro to a function

[erlend-aasland]

The macro accessed the PyTypeObject.tp_iternext member directly.
"Swap" macro variant with the function in order to hide implementation details.
(A function already existed, but it was not exposed.)

https://bugs.python.org/issue40170

[erlend-aasland]

@vstinner Would you mind reviewing this?

[vstinner]

For macOS which doesn't use LTO compiler optimization, we added private static inline functions of some "Check" functions. But I don't think that it's worth it here (I don't think that the function is commonly called in "hot code").

[erlend-aasland]

CI is done; PTAL, @vstinner.

[vstinner]

Merged, thanks.

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot PPC64LE RHEL8 LTO + PGO 3.x has failed when building commit cc54001.

What do you need to do:

1. Don't panic.
2. Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
3. Go to the page of the buildbot that failed (https://buildbot.python.org/all/#builders/458/builds/1109) and take a look at the build logs.
4. Check if the failure is related to this commit (cc54001) or if it is a false positive.
5. If the failure is related to this commit, please, reflect that on the issue and make a new Pull Request with a fix.

You can take a look at the buildbot page here:

https://buildbot.python.org/all/#builders/458/builds/1109

Summary of the results of the build (if available):

== Tests result: ENV CHANGED ==

411 tests OK.

10 slowest tests:

• test_concurrent_futures: 2 min 44 sec
• test_multiprocessing_spawn: 1 min 49 sec
• test_tokenize: 1 min 12 sec
• test_asyncio: 1 min 9 sec
• test_multiprocessing_forkserver: 1 min 2 sec
• test_multiprocessing_fork: 1 min 1 sec
• test_unparse: 50.5 sec
• test_lib2to3: 48.7 sec
• test_signal: 47.5 sec
• test_io: 38.7 sec

1 test altered the execution environment:
test_asyncio

14 tests skipped:
test_devpoll test_gdb test_ioctl test_kqueue test_msilib
test_ossaudiodev test_startfile test_tix test_tk test_ttk_guionly
test_winconsoleio test_winreg test_winsound test_zipfile64

Total duration: 4 min 31 sec

Click to see traceback logs

Traceback (most recent call last):
  File "/home/buildbot/buildarea/3.x.cstratak-RHEL8-ppc64le.lto-pgo/build/Lib/asyncio/sslproto.py", line 321, in __del__
    self.close()
  File "/home/buildbot/buildarea/3.x.cstratak-RHEL8-ppc64le.lto-pgo/build/Lib/asyncio/sslproto.py", line 316, in close
    self._ssl_protocol._start_shutdown()
  File "/home/buildbot/buildarea/3.x.cstratak-RHEL8-ppc64le.lto-pgo/build/Lib/asyncio/sslproto.py", line 590, in _start_shutdown
    self._abort()
  File "/home/buildbot/buildarea/3.x.cstratak-RHEL8-ppc64le.lto-pgo/build/Lib/asyncio/sslproto.py", line 731, in _abort
    self._transport.abort()
  File "/home/buildbot/buildarea/3.x.cstratak-RHEL8-ppc64le.lto-pgo/build/Lib/asyncio/selector_events.py", line 680, in abort
    self._force_close(None)
  File "/home/buildbot/buildarea/3.x.cstratak-RHEL8-ppc64le.lto-pgo/build/Lib/asyncio/selector_events.py", line 731, in _force_close
    self._loop.call_soon(self._call_connection_lost, exc)
  File "/home/buildbot/buildarea/3.x.cstratak-RHEL8-ppc64le.lto-pgo/build/Lib/asyncio/base_events.py", line 745, in call_soon
    self._check_closed()
  File "/home/buildbot/buildarea/3.x.cstratak-RHEL8-ppc64le.lto-pgo/build/Lib/asyncio/base_events.py", line 510, in _check_closed
    raise RuntimeError('Event loop is closed')
RuntimeError: Event loop is closed