We pass down the correct default annotation indicator to use in 100% of code rendering places, so this becomes a useless bit of initialization. We already set this to a sentinel by default, and the only thing that having an explicit str gives us is the inability to copy an annotation from a param to a return or vice versa. So, out it goes. This means we can't render Annotation by itself, so the test that was using this behavior is out too.
I added this originally because it's part of Python's grammar, but since
we merged parenthesis and friends into expressions instead of forming
separate nodes, BaseAtom doesn't have much use.
This fixes also a few bugs:
- A decorator requires a `dotted_name`, so it can only take a `Name` or
an `Attribute` node, not any atom.
- A `Call` node's function can be any expression, not just a fixed list
of `BaseExpression` subclasses.
