* Refactored to make authn modular; added GSSAPI

* Getting tests passing again

* Add missing __future__ import to new source file

* Made it more obvious that login_method/login_response not heeded

However, we could create a SASL subclass that provides the functionality
for backwards compatibility.

* Also try PLAIN if we're given a userid/password.

* Squashing flakes

* Reimplemented login_method/login_response; flake fixing

* An extra test; deflaking

* Remove a blank line to sate flake8. Great.

* SASL tests!

Also wrapped getting GSSAPI mech impl in a function, so we can test it
with and without a `gssapi` module present.

* Fixing tests

I'd misnamed my test class while debugging something, and not reverted
it. Now the new tests are being run, another mistake appeared, now
fixed.

* More tests for the coverage gods

* Attempt GSSAPI (but don't fail hard), and use userid

This adds fail_soft and client_name arguments to GSSAPI, so that:

* amqp.connection can add GSSAPI to its default list of auth mechanisms,
  but not fail if the gssapi module isn't available, or if GSSAPI
  credentials aren't available
* A user can use userid to specify which GSSAPI client credentials
  should be used.

Authentication mechanisms can also now return NotImplemented if they
decline to do SASL negotiation at start() time.

* Added another test for GSSAPI client name explicitly specified

* Make pydocstyle happy.

* Improving test coverage