Avoid creating ref cycles

[hauntsaninja]

By storing previously raised exceptions inside a local, this code created ref cycles that kept all locals in all calling stack frames alive.

This is because exceptions hold references to their tracebacks, which hold references to the relevant frames, which holds a reference to the local errors dict that holds references to the exceptions. See https://peps.python.org/pep-0344/#open-issue-garbage-collection and https://peps.python.org/pep-3110/#rationale

This breaks the cycle by deleting the local when we raise, so frames are destroyed by the normal reference counting mechanism.

This fixes some resource exhaustion issues I encountered at work.

[coveralls]

coverage: 97.888% (+0.004%) from 97.884% when pulling 8b5d2c6 on hauntsaninja:patch-1 into b29e6f4 on agronholm:master.

[agronholm]

A things of note:

1. Why not delete the errors before returning?
2. I would like to see a test that ensures there won't be reference cycles

[hauntsaninja]

Thanks for the review! I must have missed your response. I've addressed both points now :-)

[agronholm]

Would you mind also adding a note in the changelog (under **UNRELEASED**)? Then we're good.

[hauntsaninja]

Thanks, added the changelog, a fix, and a test for the fix

[agronholm]

Thanks!