system tests: Fix python crash during tests

As a developer i would like the system tests to not occasionally crash

This seems to be a python crash that takes a while to reporduce. After much testing first fixed a thread leak, but this was not source of problem. Real problem was calling an exception handler function that was a python object that had got garbage collected. We were using an earlier incantation of an exception handler replacement function that we had created, using the upstream one fixed the issue.

Start by reviewing the system-test crash reports and the exception-handler replacement described in the issue. Compare the current handler with the upstream implementation referenced in the report, then run the system tests repeatedly to check whether the intermittent Python crash is gone.

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