Update webargs to 4.0.0

[pyup-bot]

This PR updates webargs from 3.0.2 to 4.0.0.

Changelog

4.0.0

******************

Features:

* *Backwards-incompatible*: Custom error handlers receive the
`marshmallow.Schema` instance as the third argument. Update any
functions decorated with `Parser.error_handler` to take a ``schema``
argument, like so:

.. code-block:: python

  3.x
 parser.error_handler
 def handle_error(error, req):
     raise CustomError(error.messages)


  4.x
 parser.error_handler
 def handle_error(error, req, schema):
     raise CustomError(error.messages)


See `marshmallow-code/marshmallow840 (comment) <https://github.com/marshmallow-code/marshmallow/issues/840issuecomment-403481686>`_
for more information about this change.

Bug fixes:

* *Backwards-incompatible*: Rename ``webargs.async`` to
``webargs.asyncparser`` to fix compatibility with Python 3.7
(:issue:`240`). Thanks :user:`Reskov` for the catch and patch.


Other changes:

* *Backwards-incompatible*: Drop support for Python 3.4 (:pr:`243`). Python 2.7 and
>=3.5 are supported.
* *Backwards-incompatible*: Drop support for marshmallow<2.15.0.
marshmallow>=2.15.0 and >=3.0.0b12 are officially supported.
* Use `black <https://github.com/ambv/black>`_ with `pre-commit <https://pre-commit.com/>`_
for code formatting (:pr:`244`).

Links

• PyPI: https://pypi.org/project/webargs
• Changelog: https://pyup.io/changelogs/webargs/
• Repo: https://github.com/sloria/webargs

[coveralls]

Coverage remained the same at 84.95% when pulling f809222 on pyup-update-webargs-3.0.2-to-4.0.0 into 337d6b3 on master.