Parse an annotation string.
    Return an AST Expr node.
    code: annotation string (excluding '@')

def parse_annotation(code):
    """Parse an annotation string.
    Return an AST Expr node.
    code: annotation string (excluding '@')
    """
    module = ast.parse(code)
    assert type(module) is ast.Module, 'internal error #1'
    assert len(module.body) == 1, 'Annotation contains more than one expression'
    assert type(module.body[0]) is ast.Expr, 'Annotation is not expression'
    return module.body[0]