RecursionError on import when using message with a field named "str"

Hello,

We are using betterproto with the NI-XNET gRPC Server. (For Reference: https://github.com/ni/grpc-device)
They use some messages which have a field with name "str".
This causes a RecursionError on import of the generated python file.
(The recursion happens inside the dataclass library, presumabely since the str builtin gets shadowed.)

Example message spec:
```
message GetPropertyResponse {
int32 status = 1;
oneof property_value {
uint32 u32_scalar = 2;
bool bool_scalar = 3;
string str = 4;
uint64 u64_scalar = 5;
int32 i32_scalar = 6;
double f64_scalar = 7;
}
}
```

Generated python code:
```python
@dataclass
class GetPropertyResponse(betterproto.Message):
status: int = betterproto.int32_field(1)
u32_scalar: int = betterproto.uint32_field(2, group="property_value")
bool_scalar: bool = betterproto.bool_field(3, group="property_value")
str: str = betterproto.string_field(4, group="property_value")
u64_scalar: int = betterproto.uint64_field(5, group="property_value")
i32_scalar: int = betterproto.int32_field(6, group="property_value")
f64_scalar: float = betterproto.double_field(7, group="property_value")
```

Full Traceback:
```
Traceback (most recent call last):
File "C:\...\.py", line 8, in
from import
File "C:\...\.py", line 11, in
from import nixnet_grpc
File "C:\...\nixnet_grpc.py", line 1018, in
class GetPropertyResponse(betterproto.Message):
File "C:\...\Python\Python39\lib\dataclasses.py", line 1021, in dataclass
return wrap(cls)
File "C:\...\Python\Python39\lib\dataclasses.py", line 1013, in wrap
return _process_class(cls, init, repr, eq, order, unsafe_hash, frozen)
File "C:\...\Python\Python39\lib\dataclasses.py", line 993, in _process_class
str(inspect.signature(cls)).replace(' -> None', ''))
File "C:\...\Python\Python39\lib\inspect.py", line 3087, in __str__
formatted = str(param)
File "C:\...\Python\Python39\lib\inspect.py", line 2606, in __str__
formatannotation(self._annotation))
File "C:\...\Python\Python39\lib\inspect.py", line 1240, in formatannotation
return repr(annotation)
File "C:\...\Python\Python39\lib\dataclasses.py", line 260, in __repr__
return ('Field('
File "C:\...\Python\Python39\lib\dataclasses.py", line 260, in __repr__
return ('Field('
File "C:\...\Python\Python39\lib\dataclasses.py", line 260, in __repr__
return ('Field('
[Previous line repeated 482 more times]
RecursionError: maximum recursion depth exceeded
```

We are not using this field actively so I was able to resolve the issue temporarily by renaming it to "str_" in the generated files.
I have not tested whether this causes issues when actually using the field.