fix: Return coroutines instead of awaitables from functions.

[DABND19]

Callable[..., Awaitable[T]] is not a coroutine function, in fact it should be Callable[..., Coroutine[Any, Any, T]].

If we have an interface like this:

import abc

class AbstractClient(abc.ABC):
    @abc.abstractmethod
    async def fetch(self) -> bytes:
        ...

Then for such an implementation, mypy will return an error:

import aiomisc

class Client(AbstractClient):
    @aiomisc.asyncretry(max_tries=3)
    async def fetch(self) -> bytes:
        ...

We can use workaround like this:

import abc
from typing import Awaitable

class AbstractClient(abc.ABC):
    @abc.abstractmethod
    def fetch(self) -> Awaitable[bytes]:
        ...

But in this case fetch is not a coroutine function. We will have problems using unittest.mock.MagicMock:

from unittest.mock import MagicMock

mocked_client = MagicMock(AbstractClient)
mocked_client.fetch.return_value = b'Hello, World!!!'

# will raise TypeError
await mocked_client.fetch()