Methoddispatch

Handle multiple inheritance properly.
Support simple type annotations e.g. list[int] (but only use list)

Ooops - missed v4.

Drops support for plain functions.
Deprecates module register function in favour of BaseClass.method.register()
Supports per-instance dispatch registries.

* Added support for annotations as per python 3.7 [functools.singledispatch](https://docs.python.org/3/library/functools.html#functools.singledispatch) to Python 3.6+
* Use ``__init_subclass__`` instead of a meta class for Python 3.6+
* Allow using actual method instead of method name in ``register()`` function
* Fixed missing ``dispatch`` and ``registry`` attributes on method objects.