Message with oneof called method .to_pydict/.from_pydict fails

### Summary

Serializing a message with oneof fails by `to_pydict` or `from_pydict`.

### Reproduction Steps

Define a message and compile:

```protobuf
message Foo {
oneof group1 {
int32 bar = 1;
int32 baz = 2;
}
}
```

then called `Foo(bar=1).to_pydict` or `Foo().from_pydict({"bar": 1})`

### Expected Results

The method called success like `to_dict` and `from_dict`

### Actual Results

There will raise AttributeError: `'group1' is set to 'xxxx', not 'baz' ` / `'group1' is set to None, not 'bar'`:

```
Traceback (most recent call last):
File "D:\Projects\adapter-kritor\exam.py", line 22, in
res = foo1.to_pydict()
File "D:\Projects\adapter-kritor\.venv\lib\site-packages\betterproto\__init__.py", line 1717, in to_pydict
value = getattr(self, field_name)
File "D:\Projects\adapter-kritor\.venv\lib\site-packages\betterproto\__init__.py", line 836, in __getattribute__
raise AttributeError(
AttributeError: 'group1' is set to 'bar', not 'baz'
```

### System Information

libprotoc 27.2
Python 3.9.13
2.0.0b7

### Checklist

- [X] I have searched the issues for duplicates.
- [X] I have shown the entire traceback, if possible.
- [X] I have verified this issue occurs on the latest prelease of betterproto which can be installed using `pip install -U --pre betterproto`, if possible.