Replace uses of exec_ with exec

[MHendricks]

Qt6 has retired the python 2 workaround now that exec is not reserved. PyQt6 no longer supports exec_ and Qt5 already supported exec in py3. This covers the remaining uses of exec_, several others were already updated.

Checklist

• I have read the CONTRIBUTING.md document
• I formatted my changes with black
• I linted my changes with flake8
• I have added documentation regarding my changes where necessary
• Any pre-existing tests continue to pass
• Additional tests were made covering my changes

Types of Changes

• Bugfix (change that fixes an issue)
• New Feature (change that adds functionality)
• Documentation Update (if none of the other choices apply)