Log Message:
py-multidict: update to 6.0.0.

Features
--------

- Use ``METH_FASTCALL`` where it makes sense.

  ``MultiDict.add()`` is 2.2 times faster now, ``CIMultiDict.add()`` is 1.5 \ 
times faster.
  The same boost is applied to ``get*()``, ``setdefault()``, and ``pop*()`` \ 
methods. (:issue:`681`)

Bugfixes
--------

- Fixed type annotations for keys of multidict mapping classes. (:issue:`644`)
- Support Multidict[int] for pure-python version.
  ``__class_getitem__`` is already provided by C Extension, making it work with \ 
the pure-extension too. (:issue:`678`)

Deprecations and Removals
-------------------------

- Dropped Python 3.6 support (:issue:`680`)