Add test for generated service stub

- Create one simple test for generated Service stubs in preparation
for making more changes in this area.
- Add dev dependency on pytest-asyncio in order to use ChannelFor
from grpclib.testing more easily.
- Create a new example proto containing a minimal rpc example.

betterproto/tests/service.proto

@@ -0,0 +1,15 @@
+syntax = "proto3";
+
+package service;
+
+message DoThingRequest {
+  int32 iterations = 1;
+}
+
+message DoThingResponse {
+  int32 successfulIterations = 1;
+}
+
+service ExampleService {
+  rpc DoThing (DoThingRequest) returns (DoThingResponse);
+}

betterproto/tests/test_service_stub.py

@@ -0,0 +1,35 @@
+import betterproto
+import grpclib
+from grpclib.testing import ChannelFor
+import pytest
+from typing import Dict
+from .service import DoThingResponse, DoThingRequest, ExampleServiceStub
+
+
+class ExampleService:
+
+    async def DoThing(self, stream: 'grpclib.server.Stream[DoThingRequest, DoThingResponse]'):
+        request = await stream.recv_message()
+        for iteration in range(request.iterations):
+            pass
+        await stream.send_message(DoThingResponse(request.iterations))
+
+
+    def __mapping__(self) -> Dict[str, grpclib.const.Handler]:
+        return {
+            '/service.ExampleService/DoThing': grpclib.const.Handler(
+                self.DoThing,
+                grpclib.const.Cardinality.UNARY_UNARY,
+                DoThingRequest,
+                DoThingResponse,
+            ),
+        }
+
+
+@pytest.mark.asyncio
+async def test_simple_service_call():
+    ITERATIONS = 42
+    async with ChannelFor([ExampleService()]) as channel:
+        stub = ExampleServiceStub(channel)
+        response = await stub.do_thing(iterations=ITERATIONS)
+        assert response.successful_iterations == ITERATIONS