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.

certbot/setup.py

@@ -43,7 +43,6 @@ install_requires = [
     # This dependency needs to be added using environment markers to avoid its
     # installation on Linux.
     'pywin32>=300 ; sys_platform == "win32"',
-    'setuptools>=41.6.0',
 ]
 
 dev_extras = [