fix exception handling on Python 3.12

[davidhewitt]

Reworked exception handling for Python 3.12. Ref #3305

The simplifications to PyErrState result in a ~10-15% perf improvement in the bench_err benchmarks on all Python versions. On 3.12 the enum size has been reduced from 32 to 24 bytes, which might have small global perf benefits too.

While working on this I noticed a bug on <3.12 where the original panic message is not retrieved properly in PyErr::fetch if the PanicException is normalised. I've added that as a second commit.

[davidhewitt]

Pushed a more complete implementation which I'm happy with, and updated OP.

[davidhewitt]

I think best if I circle back to this later to split out into multiple PRs for the changes of behaviour / fixes etc.

[davidhewitt]

Assuming CI is green, I believe this is now good to review.

[davidhewitt]

On further reflection, I should be able to split this out to first make the PyErrState::LazyValue & PyErrState::LazyTypeAndValue -> PyErrState::Lazy change a separate PR, if that would help with reviewing.

[adamreichold]

if that would help with reviewing.

I am pretty swamped at the moment, so anything that turns this into smaller chunks would be appreciated.

[davidhewitt]

if that would help with reviewing.

I am pretty swamped at the moment, so anything that turns this into smaller chunks would be appreciated.

👍 #3323

[davidhewitt]

Ok, I think this is as small a diff as it can possibly get. I split off the second commit into #3326.