Type hints do not prevent external access to internal manager

### What is wrong

The type hints on `Service` were supposed to disallow external service consumers to call `service.manager` and hence prevent access to task scheduling functionality.

Instead external parties should be forced to gain access via `service.get_manager()` which does not expose task scheduling functionality.

Further described here:

https://github.com/ethereum/async-service/blob/850a927f4bfe0f12806f9dcc76cc24f0b8ee3a36/async_service/base.py#L36-L45

It doesn't seem to be working as demonstrated here: https://github.com/ethereum/trinity/pull/1792/files#r442016399

###

Haven't looked into it yet.

Start with async_service/base.py lines 36-45 and compare the typing behavior described in the linked Trinity pull request. Trace how Service exposes manager and how external consumers are type-checked. Done means external consumers cannot use service.manager for task scheduling, while service.get_manager() remains the supported access path.

Written by the indexing model from the issue text.