Fix serialization for FILE macro

[zbynek]

Fixes #241 (comment) by making sure the Callable is a serializable (static) class.

Testing done

Added a testcase that reproduces the problem and checked that it passes.

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue

[mwebber]

I tested this fix with the just-released 444.v52de7e9c573d (with #249) and I can confirm that it fixed the problem for me.