Bump webargs from 3.0.2 to 4.0.0

[dependabot-preview]

Bumps webargs from 3.0.2 to 4.0.0.

Changelog

Sourced from webargs's changelog.

4.0.0 (2018-07-15)

---

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**](https://github.com/parser).error_handler
def handle_error(error, req):
    raise CustomError(error.messages)


# 4.x
[**parser**](https://github.com/parser).error_handler
def handle_error(error, req, schema):
    raise CustomError(error.messages)

See marshmallow-code/marshmallow#840 (comment) <https://github-redirect.dependabot.com/marshmallow-code/marshmallow/issues/840#issuecomment-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).

Commits

• a864099 Bump version and update changelog
• 90b822f Merge pull request #250 from sloria/flask-attach-schema-to-error
• 3ead80a Pass schema instance to error handlers
• ee5ee8c Use :doc: instead of :ref: to refer to pages...
• a721931 Merge pull request #249 from sloria/pyup-update-sphinx-issues-0.4.0-to-1.0.0
• c432c2c Update sphinx-issues from 0.4.0 to 1.0.0
• dcd67a9 Merge pull request #248 from sloria/pyup-update-tox-3.1.1-to-3.1.2
• 802c434 Merge pull request #247 from sloria/pyup-update-invoke-1.0.0-to-1.1.0
• 6c946cc Update tox from 3.1.1 to 3.1.2
• 1c54e1b Update invoke from 1.0.0 to 1.1.0
• Additional commits viewable in compare view

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---

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[lipis]

@dependabot rebase.

[lipis]

@rbubley Is this safe to merge?

[rbubley]

I don't know: it passes the automated checks, but at the moment they just check that we could install gae-init and wrap the project for deployment. We don't yet have checks on whether the app can serve pages without erroring out.

Looking at the changelog though, I don't expect any of the backwards incompatibilities to hit this project.

[lipis]

Great! Let's merge it then..