[3.11] Trim trailing whitespace and test on CI (GH-104275)

[hugovk]

No description provided.

[hugovk]

I'll follow this with a combined backport of #105750 and #108212.

I left them out of this one to keep things separate and clearer.

[AlexWaygood]

A lot of this diff is removing invisible pagebreak characters. For most of these cases, we should delete the whole line rather than just stripping the "trailing whitespace" from the line:

[AlexWaygood]

See #102531 for where we removed invisible pagebreak characters from the main branch. (@hugovk, is it worth seeing if that PR could just be cleanly backported to 3.11?)

[hugovk]

Good idea, and it backported cleanly 🎉 #108266

[hugovk]

Reverted Trim trailing whitespace, will update 3.11 into here, and then see what's left to fix.

[hugovk]

And autofixed via pre-commit, let's check these changes.

[hugovk]

And applied the two suggestions which were still available.

[AlexWaygood]

LGTM now, thank you!

[hugovk]

Thank you very much for the reviews!