Add ARC support to Mailman.

Open username-removed-391087 requested to merge adityadivekar/mailman:arc into master

Add handlers for ARC, which can be enabled by the user through the configuration settings.

The handlers if enabled, are added to the BasePipeline. Specifically, the two handlers - authentication_results & sign_message, are added. The authentication_results handler is responsible for generating the preliminary Authentication Results before the message is altered by any other handlers. And the sign_message handler does the actual signing of the message, i.e. affixing the ARC-Authentication-Results, ARC-Message-Signature, ARC-Seal to the message before it goes out for delivery to the outgoing runner.

A few tests have been added to check the proper functioning of the pipelines with the new handlers. Since some of the tests require the presence of a local key for signing purposes, a test key has been created and stored in a new directory test_data inside the tests directory.

Currently the code for the two new dependencies - arc and dkim (refactored version) is not available on PyPI. As a workaround, os.system explicit calls have been added temporarily to the setup script to install these dependencies from their repository code.

@warsaw @stephenjturnbull