(improvement) remove support for protocols <3 from cython files

[mykaul]

Continued effort to remove protocol versions < 3 as was done in the native Python code.

Pre-review checklist

• [ x I have split my patch into logically separate commits.
• All commit messages clearly explain what they change and why.
• I added relevant tests for new features and bug fixes.
• All commits compile, pass static checks and pass test.
• PR description sums up the changes and reasons why they should be introduced.
• I have provided docstrings for the public items that I want to introduce.
• I have adjusted the documentation in ./docs/source/.
• I added appropriate Fixes: annotations to PR description.

[Lorak-mmk]

Thanks!