Minor perf fix in magnum-bindings buffer protocol

[Skylion007]

Motivation and Context

• My minor perf fix didn't make it into the Update Magnum to latest and adapt to API deprecation #1728 update, adding it since it's on some pretty critical code paths for converting between Magnum types and Numpy arrays.

How Has This Been Tested

• Merged as part of magnum-bindings test suite. Should have little effect as the change is mostly internal and remove accidental non-trivial copies within pybind11. Should hopefully provide a very minor perf boost.

Types of changes

• Docs change / refactoring / dependency upgrade

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have completed my CLA (see CONTRIBUTING)
• I have added tests to cover my changes.
• All new and existing tests passed.

[mosra]

I personally don't think the perf improvement would be measurable at all compared to, say, the function call dispatch, but okay :)