Fix extracting async def type and name in python mode imenu

Fix extracting async def type and name in python mode imenu

* lisp/progmodes/python.el (python-imenu--get-defun-type-name):
New function.
(python-imenu--build-tree): Use python-imenu--get-defun-type-name for
extract async or simple def type and name at current
position (Bug#24820).
* test/lisp/progmodes/python-tests.el (python-imenu-create-index-1):
(python-imenu-create-flat-index-1): Add async def's.