Ensure that stderr/stdout has flush attr before calling it

[moble]

The recent commit 63f427b (which was evidently in response to #1177) is leading to errors in some cases. It turns out that sys.stdout and sys.stderr can be None in some cases, as mentioned in the note at the bottom of this section in the python docs. (This is news to me, and the cases I've encountered this in are not like what the docs suggest.) When they are None an error occurs inside tqdm because obviously None.flush() does not exist.

This PR just checks that stderr and stdout actually have a flush attribute before using that attribute. Note that we can't just reuse fp_flush because the point of #1177 was that both stdout and stderr need to be flushed.

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For reference, I've encountered the bug on two systems:

1. Ubuntu 20.04.3 LTS with Python 3.9.6 and tqdm 4.62.2 (via github actions, for which the traceback is here)
2. Debian 4.19.160-2 with Python 3.7.3. This is through a bug report on an unrelated repo, so I can't actually check the tqdm version, but I'll bet it's >=4.61.2.

[moble]

@casperdcl Anything I can do to help move this along?

[sourcery-ai]

Sourcery Code Quality Report

❌  Merging this PR will decrease code quality in the affected files by 0.03%.

Quality metrics	Before	After	Change
Complexity	25.85 😞	25.85 😞	0.00
Method Length	71.76 🙂	72.00 🙂	0.24 👎
Working memory	15.16 😞	15.16 😞	0.00
Quality	47.27% 😞	47.24% 😞	-0.03% 👎

Other metrics	Before	After	Change
Lines	1558	1558	0

Changed files	Quality Before	Quality After	Quality Change
tqdm/std.py	47.27% 😞	47.24% 😞	-0.03% 👎

Here are some functions in these files that still need a tune-up:

File	Function	Complexity	Length	Working Memory	Quality	Recommendation
tqdm/std.py	tqdm.format_meter	73 ⛔	572 ⛔	29 ⛔	2.09% ⛔	Refactor to reduce nesting. Try splitting into smaller methods. Extract out complex expressions
tqdm/std.py	tqdm.__init__	53 ⛔	553 ⛔	39 ⛔	3.16% ⛔	Refactor to reduce nesting. Try splitting into smaller methods. Extract out complex expressions
tqdm/std.py	tqdm.pandas	53 ⛔	409 ⛔	13 😞	15.07% ⛔	Refactor to reduce nesting. Try splitting into smaller methods. Extract out complex expressions
tqdm/std.py	tqdm.pandas.inner_generator	26 😞	196 😞	13 😞	31.76% 😞	Refactor to reduce nesting. Try splitting into smaller methods. Extract out complex expressions
tqdm/std.py	tqdm.update	27 😞	154 😞	14 😞	33.57% 😞	Refactor to reduce nesting. Try splitting into smaller methods. Extract out complex expressions

Legend and Explanation

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• ⛔ very poor

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[dtrifiro]

This also happens with gitbash on Windows, see iterative/dvc#7465

[casperdcl]

Ah I think the PR description is wrong.

This PR does not fix sys.std{err,out} == None (sys.std{err,out}.write() would fail if it was really None).

I think this PR fixes libraries which monkey-patch sys.std{err,out} removing their flush() method.

[casperdcl]

thanks! Sorry for the long delay 🙏

[casperdcl]

/tag v4.63.1 165a23a