Fix erronous python error setting on failed cast

[albanD]

Description

Proposed fix for #2682 if maintainers agree that this is the right thing to do.

Fix crash in test suite for both debug cpython 3.8 and 3.9

Suggested changelog entry:

Not sure how detailed the description below should be. Let me know if this needs updating.

Prevent setting python error twice on failed argument cast

[YannickJadoul]

Cfr. #2682 (comment), this PR is more complex than the proposed workaround in #2422 (#2422 (comment)), but doesn't seem to handle all the subtleties regarding #2336. In order to fix #2682, I propose to stick to @bstaletic's minimal workaround, and fix #2336 after a more thorough investigation of the issue and its consequences.

[albanD]

I do agree that it should be throwing. I opened this PR mostly so that you could see the concrete code I was talking about.
Closing as not the right fix.

[YannickJadoul]

Perfect. Thanks for the effort and investigation! And again apologies that I didn't make the link yesterday evening (evening/night for me, at least) :-)

[albanD]

No worries!