Doc: Avoid error lexing multiprocessing docs code block on Pygments 2.15.0

[CAM-Gerlach]

Due to the changes in how tracebacks are lexed in Pygments 2.15.0 (pygments/pygments#2226 and pygments/pygments#2329 ), the following code block in the multiprocessing docs:

cpython/Doc/library/multiprocessing.rst

Lines 455 to 472 in c3cd3d1

	interactive interpreter. For example::
	>>> from multiprocessing import Pool
	>>> p = Pool(5)
	>>> def f(x):
	... return x*x
	...
	>>> with p:
	... p.map(f, [1,2,3])
	Process PoolWorker-1:
	Process PoolWorker-2:
	Process PoolWorker-3:
	Traceback (most recent call last):
	Traceback (most recent call last):
	Traceback (most recent call last):
	AttributeError: Can't get attribute 'f' on <module '__main__' (<class '_frozen_importlib.BuiltinImporter'>)>
	AttributeError: Can't get attribute 'f' on <module '__main__' (<class '_frozen_importlib.BuiltinImporter'>)>
	AttributeError: Can't get attribute 'f' on <module '__main__' (<class '_frozen_importlib.BuiltinImporter'>)>

results in a docs build error when attempting to lex the code for syntax highlighting and other features:

WARNING: Could not lex literal_block as "pycon". Highlighting skipped.

due to pygments apparently attempting to parse the traceback, which fails. This was reported in pygments/pygments#2407 , but unless and until that is fixed, we need to unbreak the docs build.

Note that the fix will need to be backported as far as 3.7 to fix the docs CIs/builds there (given that I confirmed Pygments 2.15 should be picked up even that far back), otherwise any future PRs/commits on those branches will also run into the error.

Ideally it would be nice to avoid breaking syntax highlighting, the >>> button, selection of only the user-entered text and other custom pycon features on the whole block, which could be done by splitting it into two code blocks, one with the code and one with the output. However, this would be more complex and complicate backporting, since the error is apparently different in different versions and there were other changes in between.

We could also pin to Pygments 2.14, but at the moment its unclear if they will fix it upstream (since its an extremely narrow corner case and unclear how they would distinguish it from valid cases), so it might block adoption of current and future Sphinx and pygments versions all for one single code block. This also would probably need a separate fix on the docs build server which is onerous to touch.

Therefore, I've just changed that block to use plain text as the lexer, which is slightly less ideal but is the simplest all-around solution for now, and can easily be changed/reverted later if it does get fixed upstream. Conveniently for the purpose, it also discourages the user from actually trying the example, which will hang their Python interpreter.

[carljm]

It looks like the PR currently implements the "split into two blocks" solution, which the text above rejects as "would be more complex and complicate backporting." Can you clarify which solution you intend to implement here? Is there an additional commit you haven't pushed yet?

[CAM-Gerlach]

Thanks for the catch @carljm ! Yeah oops, sorry, I'd git pushed before submitting the PR, but I didn't notice that the push had failed. Re-pushed to fix it.

[hauntsaninja]

Thank you!

[miss-islington]

Thanks @CAM-Gerlach for the PR, and @hauntsaninja for merging it 🌮🎉.. I'm working now to backport this PR to: 3.7, 3.8, 3.9, 3.10, 3.11.
🐍🍒⛏🤖

[bedevere-bot]

GH-103427 is a backport of this pull request to the 3.11 branch.

[bedevere-bot]

GH-103428 is a backport of this pull request to the 3.10 branch.

[bedevere-bot]

GH-103429 is a backport of this pull request to the 3.9 branch.

[bedevere-bot]

GH-103430 is a backport of this pull request to the 3.8 branch.

[bedevere-bot]

GH-103431 is a backport of this pull request to the 3.7 branch.

[hugovk]

Pygments 2.15.1 has been released fixing the error that required this workaround.

• https://github.com/pygments/pygments/releases/tag/2.15.1
• Traceback parsing changes in 2.15.0 error on valid pycon code blocks pygments/pygments#2407 (comment)

Please see PR GH-103616 to revert this.