[3.8] bpo-39562: Prevent collision of future and compiler flags (GH-19230)

[pablogsal]

The constant values of future flags in the future module
is updated in order to prevent collision with compiler flags.
Previously PyCF_ALLOW_TOP_LEVEL_AWAIT was clashing
with CO_FUTURE_DIVISION..
(cherry picked from commit 4454057)

Co-authored-by: Batuhan Taşkaya batuhanosmantaskaya@gmail.com

https://bugs.python.org/issue39562

[pablogsal]

We need to still maybe add a better way to document this? @vstinner Where would you like for the documentation for this change to live?

[vstinner]

We need to still maybe add a better way to document this? @vstinner Where would you like for the documentation for this change to live?

Can you at least backport the NEWS entry?

[vstinner]

LGTM if you add the NEWS entry.

I tested manually: this change does fix https://bugs.python.org/issue39562#msg365311

[vstinner]

We need to still maybe add a better way to document this? @vstinner Where would you like for the documentation for this change to live?

One option is to add a "Notable changes in Python 3.8.3" section since the change is backward incompatible. Something like:
https://docs.python.org/dev/whatsnew/3.8.html#notable-changes-in-python-3-8-1

But it's up to you to decide if it's worth it or not. The 3.9 change is documented in What's New in Python 3.9, in the "Porting to Python 3.9" section:
https://docs.python.org/dev/whatsnew/3.9.html#changes-in-the-python-api

[pablogsal]

Oh, seems like @isidentical used the new | operator on dict:

======================================================================
ERROR: test_ensure_flags_dont_clash (test.test_future.FutureTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/travis/build/python/cpython/Lib/test/test_future.py", line 88, in test_ensure_flags_dont_clash
    flags |= {
TypeError: unsupported operand type(s) for |=: 'dict' and 'dict'
----------------------------------------------------------------------

I will update the PR soon to solve this and with @vstinner suggestions

[miss-islington]

@pablogsal: Status check is done, and it's a success ✅ .