Match python task append/prepend syntax

[wesQ3]

Currently an error label is displayed on python functions with :append/prepend. See this example from the user manual:

example.bb

```bitbake python some_python_function () { d.setVar("TEXT", "Hello World") print d.getVar("TEXT") } python do_foo:prepend() { bb.plain("first") } python do_foo() { bb.plain("second") } python do_foo:append() { bb.plain("third") } ```