bpo-42955: Remove sub-packages from sys.stdlib_module_names

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https://bugs.python.org/issue42955

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Tests / Ubuntu (pull_request) Failing

This is unrelated to my PR, FAIL: test_identify (tkinter.test.test_ttk.test_widgets.ComboboxTest).
https://bugs.python.org/issue42507

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cc @stevendaprano @pablogsal

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@gaborbernat @warsaw @gpshead: What do you think of this change?

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To me, this makes sense. The use case we've had at work for an official list of what modules are in the standard library has been in code health tooling. Sorting stdlib imports before others, etc. Only the top level package names are important there.

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Merged, thanks @gpshead.

Well, I wasn't fully comfortable with the half-baken trade-off: include sub-packages but not sub-modules. IMO the new API is now more consistent: only list main packages.

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⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot x86 Gentoo Installed with X 3.x has failed when building commit 64fc105.

What do you need to do:

1. Don't panic.
2. Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
3. Go to the page of the buildbot that failed (https://buildbot.python.org/all/#builders/464/builds/742) and take a look at the build logs.
4. Check if the failure is related to this commit (64fc105) or if it is a false positive.
5. If the failure is related to this commit, please, reflect that on the issue and make a new Pull Request with a fix.

You can take a look at the buildbot page here:

https://buildbot.python.org/all/#builders/464/builds/742

Summary of the results of the build (if available):

Click to see traceback logs

remote: Not Found
fatal: repository 'https://github.com/python/cpython.git/' not found

chmod: cannot access 'target/': No such file or directory

make: *** No rule to make target 'distclean'.  Stop.