Match only_rerun against wrapped exception causes

[teddytennant]

Fixes #353.

--only-rerun / only_rerun (and --rerun-except) match the outermost exception only. A transient error wrapped with raise ... from is not retried, even when the inner type is exactly what the filter is for.

def test_wrapped():
    try:
        raise MemoryError("out of memory")
    except MemoryError as error:
        raise RuntimeError("something failed") from error

$ pytest test_wrapped.py --reruns 2 --only-rerun MemoryError
1 failed            # not rerun

The same test without the wrapper reruns as expected. _try_match_error built the match string from excinfo.type / excinfo.value and never walked __cause__ or __context__.

The matcher now walks that chain (skipping a context suppressed by raise ... from None) and treats a hit on any linked exception as a match. Existing outer-exception matches are unchanged.

Regression tests cover a __cause__ wrap, an implicit __context__ wrap, the exception-class form of only_rerun, rerun_except on a wrapped error, a suppressed context, and a non-matching control.

$ pytest tests/test_pytest_rerunfailures.py -k wrapped
7 passed
$ pytest tests/
201 passed, 7 skipped