Update cache.py

[peterdudfield]

Pull Request

Description

Copy cache items before iterating

Helps with #186

How Has This Been Tested?

CI tests

• Yes

Checklist:

• My code follows OCF's coding style guidelines
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I have checked my code and corrected any misspellings

[sentry-io]

🔍 Existing Issues For Review

Your pull request is modifying functions with the following pre-existing issues:

📄 File: pv_site_api/cache.py

Function	Unhandled Issue
remove_old_cache	RuntimeError: dictionary changed size during iteration /sites/{site_uuid}/pv_fo... Event Count: 10
remove_old_cache	RuntimeError: dictionary changed size during iteration /sites/pv_... Event Count: 1

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