Adds example of accessing VCR Cassette object in tests

[dsummersl]

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Description

Adds an example showing how to access the VCR Cassette ojbect in a pytest (https://vcrpy.readthedocs.io/en/latest/advanced.html)

Checklist

• [-] Created tests which fail without the change (if possible)
• [x] All tests passing
• [x] Added a changelog entry
• [x] Extended the README / documentation, if necessary

[Stranger6667]

Thank you!