Add mock_async_callable for asyncio support

[jivid]

Add a mock_async_callable DSL which works exactly like mock_callable but for async functions.

The decision to not patch mock_callable to support async functions was intentional and made to make it clear when reading the test what kind of code is being mocked. Just as you can't just stick async in front of a function and expect everything to work, you shouldn't rely on mock_callable just working either.

[fornellas]

The logic for __new__ is duplicated, it could be moved to _MockCallableDSL.

[fornellas]

Neat! This seems in a good direction! There are a few things that we need to look over as well:

• Documentation needs to be updated to explain about async usage (including test framework integration) . Tip: do make docs to build and test locally.
• mock_callable() supports a lot of different configurations. I don't expect them all to work with the current implementation, we need tests for all of them.

From the top of my mind, I do expect these to "just work":

• .to_return_value(value)
• .to_return_values(values_list)
• .to_raise(exception)
• .and_assert_called_exactly(times)
• .and_assert_called_once()
• .and_assert_called_twice()
• .and_assert_called_at_least(times)
• .and_assert_called_at_most(times)
• .and_assert_called()
• .and_assert_called_ordered()
• .and_assert_not_called()

But I have my doubts about these:

• .for_call(*args, **kwargs)
• .to_yield_values(values_list)
• .with_implementation(func)
• .with_wrapper(func)
• .to_call_original()

Unfortunately Python has shown to be full of quicks and exception cases on how things work. The current tests, while hard to maintain and confusing (sorry about that), were extremely helpful in getting all corner cases in the past, we'd loose a lot if we did not have the same coverage for async.

Ideally, we'd need to duplicate all sync wyth async equivalents of the tests, but in some cases (eg: iterator, calling original) we'd need more async specific stuff. I'm happy to assist you on that if needed.

[jivid]

Thanks for the comments @fornellas, I'll update this PR with the requested changes

[jivid]

@fornellas I'm still working on this, not ready for review yet. Just pushed my changes so far so I don't lose them.

[fornellas]

FYI, #59 should help testing async stuff, I'm still working on it.

[fornellas]

@jivid as we spoke, I'm gonna start working on this issue now so we can deliver it ASAP. I'm closing this PR so we don't clash, but feel free to ping me to join forces if you can spare bandwidth.

Thanks for what we have here already, I have something to grow on top :-D