Drop setuptools as a runtime dependency

Because of the change from using setuptools.pkg_resources to using
importlib, we no longer need a runtime dependency on setuptools. It is
still required, however, for running setup.py.

acme/setup.py

@@ -15,7 +15,6 @@ install_requires = [
     'pyrfc3339',
     'pytz>=2019.3',
     'requests>=2.20.0',
-    'setuptools>=41.6.0',
 ]
 
 docs_extras = [