Fix crash if a callable returning a context manager was assigned to a list or dict item

[DudeNr33]

Steps

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Description

#4721 introduced a crash if a context manager was not assigned to a variable or attribute, but instead to something like a list or dictionary item.

_append_context_managers_to_stack() now ignores assignments where the left hand side is not assigned to something with a name. This will lead to consider-using-with to be triggered when visit_call() is called.
Note that this would have been the same behaviour as before #4721, so this is not a step back.

Of course it would be desirable if it is possible to properly detect if the list/dict item is extracted later on and used in a with block, like so:

import subprocess

job_dict = {}
job_dict["new"] = subprocess.Popen("ls")  # will currently emit consider-using-with

# ... 

for name, job in job_dict.items():
    with job:  # we now actually use the context manager, thus it would not have been necessary to emit consider-using-with earlier
        output = job.communicate()

If anybody has an idea how to detect this, I'll be happy to implement this.

I'm also guessing that this would probably go in a 2.9.5? If not I'll change the ChangeLog and whatsnew entry again.

Type of Changes

	Type
✓	🐛 Bug fix

Related Issue

Closes #4732

[coveralls]

Coverage remained the same at 92.09% when pulling 6833c32 on DudeNr33:fix-4732 into 970c5d2 on PyCQA:main.

[Pierre-Sassoulas]

I think not handling the false positive is fine for now as we're aiming to suppress the crash first. Thank you for this fix 👍