The split enum value types break type checks when used with dataclasses

Changes made in https://github.com/dropbox/mypy-protobuf/pull/120 are breaking typechecks when compiled types are used with dataclasses, or in general when enums are used as parameters or return values.

Quick example: proto:

syntax = "proto3";

package bikes;
    
enum EngineType {
    TWO_STROKE = 0;
    FOUR_STROKE = 1;
}

message Motorcycle {
    string name = 1;
    EngineType engine_type = 2;
}

python:

from dataclasses import dataclass
from motorcycles_pb2 import Engine, EngineType, Motorcycle

@dataclass
class DemoEngine:
    engine_type: EngineType
    name: str

    def __init__(self, engine_type: EngineType, name: str):
        self.engine_type = engine_type
        self.name = name

d = DemoEngine(EngineType.TWO_STROKE, "demo")

type check failure:

motorcycles/__main__.py:17: error: Argument 1 to "DemoEngine" has incompatible type "EngineTypeValue"; expected "EngineType"

Protos compiled with protoc 3.12.2 using python -m grpc_tools.protoc

[pcorpet]

@nipunn1313 In your PR you’re only using comments for types. As such all you use are strings (they are never considered as actual Python object). What I use is full typing annotations but using strings:

e: 'test_pb2.OuterEnumValue' = test_pb2.BAR

Previously I was able to use without the quotes but not anymore

e: test_pb2.OuterEnum = test_pb2.BAR

[nipunn1313]

cool - I will close this out and make a task to add a py3-syntax test.