Remove FastAPI from prefect.flow's default import path. (#16473)

src/prefect/flows.py

@@ -37,7 +37,6 @@
 from uuid import UUID
 
 import pydantic
-from fastapi.encoders import jsonable_encoder
 from pydantic.v1 import BaseModel as V1BaseModel
 from pydantic.v1.decorator import ValidatedFunction as V1ValidatedFunction
 from pydantic.v1.errors import ConfigError  # TODO
@@ -613,6 +612,8 @@ def serialize_parameters(self, parameters: dict[str, Any]) -> dict[str, Any]:
                 serialized_parameters[key] = f"<{type(value).__name__}>"
                 continue
             try:
+                from fastapi.encoders import jsonable_encoder
+
                 serialized_parameters[key] = jsonable_encoder(value)
             except (TypeError, ValueError):
                 logger.debug(