Fix editable install in CI

Description

Setuptools 64 changed how editable installs work. In the new scheme, the setuptools build_ext command thingy is needed for editable installs with extension modules to work, but we were telling it to use the Cython one. This seems to be unnecessary, as the Cython docs show examples without overriding build_ext.

How Has This Been Tested?

CI passes 😉

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the code style of this project.

Reviewers

@roscar @schmidtp