Update attrs to 19.1.0

[pyup-bot]

This PR updates attrs from 18.2.0 to 19.1.0.

Changelog

19.1.0

-------------------

Backward-incompatible Changes
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

- Fixed a bug where deserialized objects with ``cache_hash=True`` could have incorrect hash code values.
This change breaks classes with ``cache_hash=True`` when a custom ``__setstate__`` is present.
An exception will be thrown when applying the ``attrs`` annotation to such a class.
This limitation is tracked in issue `494 <https://github.com/python-attrs/attrs/issues/494>`_.
`482 <https://github.com/python-attrs/attrs/issues/482>`_


Changes
^^^^^^^

- Add ``is_callable``, ``deep_iterable``, and ``deep_mapping`` validators.

* ``is_callable``: validates that a value is callable
* ``deep_iterable``: Allows recursion down into an iterable,
 applying another validator to every member in the iterable
 as well as applying an optional validator to the iterable itself.
* ``deep_mapping``: Allows recursion down into the items in a mapping object,
 applying a key validator and a value validator to the key and value in every item.
 Also applies an optional validator to the mapping object itself.

You can find them in the ``attr.validators`` package.
`425 <https://github.com/python-attrs/attrs/issues/425>`_
- Fixed stub files to prevent errors raised by mypy's ``disallow_any_generics = True`` option.
`443 <https://github.com/python-attrs/attrs/issues/443>`_
- Attributes with ``init=False`` now can follow after ``kw_only=True`` attributes.
`450 <https://github.com/python-attrs/attrs/issues/450>`_
- ``attrs`` now has first class support for defining exception classes.

If you define a class using ``attr.s(auto_exc=True)`` and subclass an exception, the class will behave like a well-behaved exception class including an appropriate ``__str__`` method, and all attributes additionally available in an ``args`` attribute.
`500 <https://github.com/python-attrs/attrs/issues/500>`_
- Clarified documentation for hashing to warn that hashable objects should be deeply immutable (in their usage, even if this is not enforced).
`503 <https://github.com/python-attrs/attrs/issues/503>`_


----

Links

• PyPI: https://pypi.org/project/attrs
• Changelog: https://pyup.io/changelogs/attrs/
• Homepage: https://www.attrs.org/

[pyup-bot]

Closing this in favor of #222