Update kombu to 4.2.1

[pyup-bot]

This PR updates kombu from 4.1.0 to 4.2.1.

Changelog

4.2.1

=====
:release-date: 2018-05-21 09:00 A.M IST
:release-by: Omer Katz

.. .. note::

The 4.2.0 release contained remains of the ``async`` module by accident.
This is now fixed.

- Handle librabbitmq fileno raising a ValueError when socket is not connected.

Contributed by **Bryan Shelton**

.. _version-4.2.0:

4.2.0

=====
:release-date: 2018-05-21 09:00 A.M IST
:release-by: Omer Katz

- Now passing ``max_retries``, ``interval_start``, ``interval_step``,
``interval_max`` parameters from broker ``transport_options`` to
:meth:`~kombu.Connection.ensure_connection` when returning
:meth:`~kombu.Connection.default_connection` (Issue 765).

 Contributed by **Anthony Lukach**.

- Qpid: messages are now durable by default

 Contributed by **David Davis**

- Kombu now requires version 2.10.4 or greater of the redis library,
in line with Celery

 Contributed by **Colin Jeanne**

- Fixed ImportError in some environments with outdated simplejson

 Contributed by **Aaron Morris**

- MongoDB: fixed failure on MongoDB versions with an "-rc" tag

 Contributed by **dust8**

- Ensure periodic polling frequency does not exceed timeout in
virtual transport

 Contributed by **Arcadiy Ivanov**

- Fixed string handling when using python-future module

 Contributed by **John Koehl"

- Replaced "async" with "asynchronous" in preparation for Python 3.7

 Contributed by **Thomas Achtemichuk**

- Allow removing pool size limit when in use

 Contributed by **Alex Hill**

- Codebase improvements and fixes by:

 - **j2gg0s**
 - **Jon Dufresne**
 - **Jonas Lergell**
 - **Mads Jensen**
 - **Nicolas Delaby**
 - **Omer Katz**

- Documentation improvements by:

 - **Felix Yan**
 - **Harry Moreno**
 - **Mads Jensen**
 - **Omer Katz**
 - **Radha Krishna. S.**
 - **Wojciech Matyśkiewicz**

.. _version-4.1.0:

Links

• PyPI: https://pypi.org/project/kombu
• Changelog: https://pyup.io/changelogs/kombu/
• Docs: https://kombu.readthedocs.io