[Bug] StreamListener crashes when `_output_type` is a Generic

[clxyder]

What happened?

StreamListener crashes when _output_type is a Generic with the following error message:

TypeError: issubclass() arg 1 must be a class

Steps to reproduce

MNE

import asyncio
import dspy

dspy.configure(lm=dspy.LM("openai/gpt-4o-mini", cache=False))

# Create the program and wrap it with streaming functionality
class Hello(dspy.Signature):
    world: List[str] = dspy.OutputField()

predict = dspy.Predict(Hello)
stream_listeners = [
    dspy.streaming.StreamListener(signature_field_name="world"),
]
stream_predict = dspy.streamify(predict, stream_listeners=stream_listeners)

async def use_streaming():
    output = stream_predict(
        question="why did a chicken cross the kitchen?",
        include_final_prediction_in_output_stream=False,
    )
    return_value = None
    async for value in output:
        if isinstance(value, dspy.Prediction):
            return_value = value
        else:
            print(value)
    return return_value

output = asyncio.run(use_streaming())
print(output)

this happens because issubclass expects a non-generic class.

dspy/dspy/streaming/streaming_listener.py

Line 138 in d42f29e

if self._output_type and issubclass(self._output_type, Type) and self._output_type.is_streamable():

the following snippet shows the core components at work:

import dspy
from typing import List
    
class Hello(dspy.Signature):
    world: List[str] = dspy.OutputField()

t=Hello.output_fields["world"].annotation
print(f"{t=}")

# Only call issubclass if t is actually a class
import inspect
if inspect.isclass(t):
    issubclass(t, str)
else: print("not a class")

it seem that an easy way to fix the issue would be to check if self._output_type is an instantiable class before running issubclass.

i'm bypassing that check entirely by overloading the _output_type property with type(str)

DSPy version

3.1.0b1