When multiple singledispatch decorators are stacked, the first typehints
are copied to the subsequent definitions unexpectedly.
Now autodoc generates a dummy function not to affect typehints to
subsequent functions.
Keep imports alphabetically sorted and their order homogeneous across
Python source files.
The isort project has more feature and is more active than the
flake8-import-order plugin.
Most issues caught were simply import ordering from the same module.
Where imports were purposefully placed out of order, tag with
isort:skip.
If a singledispatch function has its `__signature__` rewritten, autodoc
fails to annotate that signature because `func.__annotations__` is not
consulted. Instead, directly assign to `__signature__` instead.
