fix for issue #815

[sgtBurns7]

Description

added a fallback strategy to _search_in_f_string in occurences.py allowing for backwards compatibility as the initial parsing method will be the same, added unit test case for the bug provided

Fixes #(issue)

Checklist (delete if not relevant):

• [Done] I have added tests that prove my fix is effective or that my feature works
• [ ] I have updated CHANGELOG.md
• [ ] I have made corresponding changes to user documentation for new features
• [ ] I have made corresponding changes to library documentation for API changes