Have Python mode cooperate asynchronously with Eldoc

Have Python mode cooperate asynchronously with Eldoc

When combined with Flymake mode, which also adds a value to
eldoc-documentation-functions, Python-mode users can now experiment
with different eldoc-documentation-strategy values.

Also, this shoulda allow us to write automatic tests for this
particular Eldoc functionality.

* lisp/progmodes/python.el (inferior-python-mode): Set
coming-preoutput-filter-functions.
(python--shell-output-filter-in-progress)
(python--shell-output-filter-buffer): Rename from python- variant.
(python-shell-output-filter): Rework to support async operation.
(python-eldoc--get-doc-at-point): Rework to support async.
(python-eldoc-function): Use callback.