source.py: don't let StopIteration propagate to silence() contextmanager

Benjamin Schubertrequested to merge
bschubert/fix-silence-stopiteration-pep-0479 into master

Description

Starting from python 3.7, raising a StopIteration from a context manager will be converted to a RuntimeError, as per PEP479.

This MR fixes all occurrences of this warning.

An example that shows the warnings is https://gitlab.com/BuildStream/buildstream/-/jobs/117989712. It can be checked on the current pipeline that this does not arise anymore

Changes proposed in this merge request:

  • Don't let StopIteration propagate to the silence context manager

This merge request, when approved, will close:

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