Check that sys.stdout/err is not None before flushing

[jenshnielsen]

On Windows it seems that sys.stdout and possibly sys.stderr can be None.
This happens at startup of the kernel in Spyder e.g
spyder-ide/spyder#7842 and multiple duplicates.

The change to flush was introduced in 4.9 with #314

Checking that setout/stderr is not None before flushing seems to be enough to fix the issue for me.

[ccordoba12]

Thanks a lot @jenshnielsen! It looks good to me.

[Carreau]

Marking as 4.10, the current master branch have diverged from 4.x, so this will need a backport.

[lumberbot-app]

Something went wrong ... Please have a look at my logs.

[Carreau]

@meeseeksdev backport to 4.x

[lumberbot-app]

Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

$ git checkout 4.x
$ git pull

2. Cherry pick the first parent branch of the this PR on top of the older branch:

$ git cherry-pick -m1 acb1e39700ed33b4ecd62bc20c18172a3bbe1fa6

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

$ git commit -am 'Backport PR #344: Check that sys.stdout/err is not None before flushing'

4. Push to a named branch :

git push YOURFORK 4.x:auto-backport-of-pr-344-on-4.x

5. Create a PR against branch 4.x, I would have named this PR:

"Backport PR #344 on branch 4.x"

And apply the correct labels and milestones.

Congratulation you did some good work ! Hopefully your backport PR will be tested by the continuous integration and merged soon!

If these instruction are inaccurate, feel free to suggest an improvement.

[Carreau]

branches were messed up, re-trying.

[Carreau]

@meeseeksdev backport to 4.x