Issue #11850 - Add sys.last_exc

[robotherapist]

Closes #11850

[bluetech]

Thanks for taking this @robotherapist, looks good to me.

I do wonder about the "Skip this frame" thing, however I don't think we should be touching the exc traceback itself, so it's probably OK.

A couple of things before we can merge this:

• Add a changelog entry
• Add a test -- can add a check for last_exc to test_store_except_info_on_error in testing/test_runner.py.

[robotherapist]

@bluetech Does this require updating mypy? The error in pre-commit.ci is:

ruff.....................................................................Passed
ruff-format..............................................................Passed
trim trailing whitespace.................................................Passed
fix end of files.........................................................Passed
fix python encoding pragma...............................................Passed
check yaml...............................................................Passed
debug statements (python)................................................Passed
blacken-docs.............................................................Passed
setup-cfg-fmt............................................................Passed
type annotations not comments............................................Passed
mypy.....................................................................Failed
- hook id: mypy
- exit code: 1

src/_pytest/runner.py:172: error: Module has no attribute "last_exc"  [attr-defined]
src/_pytest/runner.py:182: error: Module has no attribute "last_exc"  [attr-defined]
testing/test_runner.py:930: error: Module has no attribute "last_exc"  [attr-defined]
Found 3 errors in 2 files (checked 232 source files)

rst......................................................................Passed
changelog filenames..................................(no files to check)Skipped
py library is deprecated.................................................Passed
py.path usage is deprecated..............................................Passed

Weird it ran the first time though... I was hoping the if statements for 3.12.0 would satisfy everything.

[bluetech]

LGTM, thanks! See my comment about the mypy issue.

Two "procedural" notes, in case you'd like to submit more PRs in the future:

• Prefer to update the branch of PRs using git rebase, not git merge, this makes it a bit easier to handle.
• Prefer to submit PRs from a branch in your fork, not from the main branch. This allows maintainers to update the PR if needed, and probably cause less of a mess for you as the submitter.

[bluetech]

Since the changelog is intended for end-users to read, they don't really need to know about details like pytest_runtest_call or the internal file it's done in. I suggest something like this:

Suggested change

	Added the new `sys.last_exc` value to `pytest_runtest_call()` in `src/_pytest/runner.py`.
	Added support for :data:`sys.last_exc` for post-mortem debugging on Python>=3.12.

[bluetech]

Turns it the type stub for it is just missing from typeshed (the project which provides type annotations for the standard library). I sent a fix for it, but until it's integrated (in a future mypy version), let's add type-ignores for it, like this:

Suggested change

	del sys.last_exc
	del sys.last_exc # type: ignore[attr-defined]

[bluetech]

Merged through #12027, thanks @robotherapist.