Add setuptools as an explicit build requirement

From the [virtualenv user guide](https://virtualenv.pypa.io/en/latest/user_guide.html#seeders):
>  Installing setuptools and wheel is disabled by default on Python 3.12+ environments.

This is just explicitly requiring setuptools that has otherwise been
provided by default. While we may be "saved" for a bit longer yet by
transitive dependency on setuptools (e.g. via setuptools_scm), better to
be explicit. (And even setuptools_scm is likely to drop the dependency,
actually, according to @henryiii)

Author: ksunden

pyproject.toml

@@ -4,6 +4,7 @@ requires = [
     "certifi>=2020.06.20",
     "oldest-supported-numpy",
     "pybind11>=2.6",
+    "setuptools>=42",
     "setuptools_scm>=7",
 ]