Add runtime dependency on setuptools

setuptools_rust has a runtime dependency on setuptools. Ensure that all
installations of setuptools_rust have a sufficiently recent version of
setuptools.

Fixes: #101
Signed-off-by: Christian Heimes <christian@python.org>

setup.cfg

@@ -27,7 +27,7 @@ classifiers =
 [options]
 packages = setuptools_rust
 zip_safe = True
-install_requires = semantic_version>=2.6.0; toml>=0.9.0
+install_requires = setuptools>=46.1; semantic_version>=2.6.0; toml>=0.9.0
 setup_requires = setuptools>=46.1; wheel; setuptools_scm[toml]>=3.4.3
 
 [options.entry_points]