bpo-13236: Flush the output stream more often in unittest

[serhiy-storchaka]

It can prevent some losses when output to buffered stream.

https://bugs.python.org/issue13236

[arhadthedev]

Would it be better to replace all self.stream.flush() with -u command line flag in TESTPYTHON/TESTRUNNER?

$ python -h
[...]
-u     : force the stdout and stderr streams to be unbuffered;
         this option has no effect on stdin; also PYTHONUNBUFFERED=x
[...]

[iritkatriel]

LGTM.

[serhiy-storchaka]

'-u' only affects sys.stdout and sys.stderr. unittest.TextTestRunner can be used with other streams.

Also -u can affect behavior of the tested code.

And it is not only about running Python tests. unittest is used in third-party code.

[miss-islington]

Thanks @serhiy-storchaka for the PR 🌮🎉.. I'm working now to backport this PR to: 3.9, 3.10.
🐍🍒⛏🤖

[miss-islington]

Sorry, @serhiy-storchaka, I could not cleanly backport this to 3.10 due to a conflict.
Please backport using cherry_picker on command line.
cherry_picker f42a06ba279c916fb67289e47f9bc60dc5dee4ee 3.10

[miss-islington]

Sorry @serhiy-storchaka, I had trouble checking out the 3.9 backport branch.
Please backport using cherry_picker on command line.
cherry_picker f42a06ba279c916fb67289e47f9bc60dc5dee4ee 3.9

[bedevere-bot]

GH-29929 is a backport of this pull request to the 3.10 branch.