PEP 670: clarify cast; don't change return type

[vstinner]

• Clarify how arguments are cast
• Limited C API 3.11 no longer cast arguments
• No longer remove return value
• Require to not change the return type
• Don't change macros having multiple return types

[vstinner]

I updated PEP 670 to take in account @encukou and @gpshead feedback on python-dev and on my python/cpython#31221 review.

@encukou@gpshead@erlend-aasland: Would you mind to have a look?

[vstinner]

You can read the formatted PEP by clicking in the Files Changes tab on the 3 middle dots (...) menu at the pep-0670.rst line: in this menu, click on "View file".

Current link: pep-0670.rst.

[CAM-Gerlach]

Fixes for a number of grammar errors in the added/changed text, as well as a few PEP 12 issues and for correct code block syntax highlighting. All the changes are as suggestions, and can be applied cleanly together by simply going to the Files tab, clicking the Add to batch button on each change, and then clicking the Commit button, which will avoid the all problems we both experienced last time. Alternatively, I'm happy to do it for you. Either way, don't forget to pull your branch after doing so. Thanks!

[CAM-Gerlach]

Suggested change

	For example, Python 3.7 changed ``PyUnicode_AsUTF8()`` return type from
	For example, Python 3.7 changed ``PyUnicode_AsUTF8()``\ 's return type from

Fix grammar error

[CAM-Gerlach]

This grammar error wasn't fixed

[erlend-aasland]

I'm fine with this, Victor.

[vstinner]

@CAM-Gerlach: I addressed most of your reviews, thanks.

[vstinner]

Follow-up: I wrote an email to python-dev: https://mail.python.org/archives/list/python-dev@python.org/thread/VM6I3UHVMME6QRSUOYLK6N2OZHP454W6/

[CAM-Gerlach]

Good call on using the language:: c directive in lieu of .. code-block:: c on all the code blocks, but it looks like at least a few of the grammar errors cited were not fixed (FYI, you can edit them, e.g. to remove the code-block directive, and apply those instead, to avoid them getting missed in the future). If I do get the chance to do a full copyedit of the PEP (I've been busy with other PEP editor stuff, sorry), I'll clean those up.