Memory leak fix

What does this MR do and why?

Fixes an issue with Py_XDECREF and Py_XINCREF that leads to a memory leak. Probably the solution to #626 (closed)

• fixing incorrect operator leading to memory leak

• trying single different elements, seems to be a mirror/bs/lens issue?

• src/finesse/cyexpr.pyx

adding free

• adding memory leak test

How to set up and validate locally

New test tests/functional/test_memory_leaks.py

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• Includes tests for added/modified code
• Added or modified the documentation if relevant
• Docstrings added/modified for new/modified functions
• Committer/reviewer has ran the documentation stage when ready to merge