Missing dependency message does not appear when it should

Thanks for the great work on `cloudpathlib`! I noticed a bug in the documentation for using `no_sign_request=True` [found here](https://cloudpathlib.drivendata.org/stable/authentication/#accessing-public-s3-buckets-without-credentials). When I attempt to use the example code, an error shows: `NameError: name 'Session' is not defined`. Here's an example reproducing the error: https://gist.github.com/d33bs/eb19f864baae89757e0e0969611cf1af