Allow attribute definition in any method

Currently attributes can be defined by assigning via self in the `__init__` method. This should be extended to all methods, in order to improve compatibility with Python:

```
class A:
    def __init__(self) -> None:
        self.x = 1 # Ok, define x
    def foo(self) -> None:
        self.y = 'a' # Ok, define y
```
