Fix deadlock that is maybe causing test to occasionally fail in PythonChannelTest

Description of work

Following on from #36326 I found another place that a deadlock can occur that is most probably the reason for the test failures as related to #37366. The changes in #37427 didn't fully address the issue.

We need to release GIL before doing ConfigService::getString as it will log to debug if the key doesn't exist and this is called in simpleapi.py. You may need to read the description from #36326 to fully understand the issue.

Summary of work

Fixes #xxxx.

Further detail of work

To test:

It is hard to reproduce the failing test, I ran the following for over an hour for it to happen. This will continuously run the command until it fails.

while ./systemtest -R PythonChannel --quiet --output-on-failure ; do :; done

This does not require release notes because this issue hasn't been observed by users and is only to stabilize a test

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mantidproject / mantid