None of this is ideal, since we're making the dependencies tighter than they theoretically need to be, but the behavior of the old le-auto makes this necessary to make it succeed in practice (when using LE wheels).
Once we move to the new le-auto (which pins everything and makes setup.py dependencies irrelevant for auto installs), we should redo this using env markers as in https://github.com/letsencrypt/letsencrypt/pull/2177. We're too afraid to do it now.
Similarly, we're too afraid to change how we handle argparse right now, despite that it should be required directly by us under 2.6. In practice, ConfigArgParse pulls it in for us, so we're okay as long as it continues to do that.
Erik Rose