`readable_stubs` generates invalid files for GRPC

[KapJI]

When using --mypy_grpc_out=readable_stubs:. async and sync imports are duplicated and type arguments clash:

from grpc import (
    Channel,
    Server,
    ServicerContext,
    UnaryStreamMultiCallable,
    UnaryUnaryMultiCallable,
)
from grpc.aio import (
    Channel,
    Server,
    ServicerContext,
    UnaryStreamMultiCallable,
    UnaryUnaryMultiCallable,
)

class ServiceStub:
    def __init__(self, channel: Union[Channel, Channel]): -> None: ...