Drop the Cython version check from setup.py

[musicinmybrain]

This avoids the dependency on the pkg_resources module, which has long been deprecated and is no longer shipped with current versions of setuptools.

---

The version check could have preserved by replacing

           import pkg_resources
           # …
           cython_dep = pkg_resources.Requirement.parse(CYTHON_DEPENDENCY)
           if Cython.__version__ not in cython_dep:

with

           from packaging.requirements import Requirement
           # …
           cython_dep = Requirement(CYTHON_DEPENDENCY)
           if not cython_dep.specifier.contains(Cython.__version__):

but then we would need to add a dependency on packaging to build-system.requires in pyproject.toml, and if we can trust build-system.requires to give us packaging, then we should be able to trust it to give us the correct version of Cython, too!