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.

Pipfile

@@ -8,6 +8,7 @@ flake8 = "*"
 mypy = "*"
 isort = "*"
 pytest = "*"
+pytest-asyncio = "*"
 rope = "*"
 v = {editable = true,version = "*"}