Add Python 3.5, 3.6 and 3.12.0-rc.1 to test matrix

[fcollonval]

Closes #762

This adds:

• test against Python 3.5, 3.6 and 3.12.0-rc.1

[fcollonval]

@Red-M all tests are green on the additional Python version; see fcollonval#1

Is there something more to do? Are there tests in tests/test_async.py missing?

[Red-M]

Yes, there was new features added in #732 and they need to have tests written for them

[fcollonval]

Thanks @Red-M for the reply - I'm gonna need some help to be sure to get it right.

Looking at the coverage, there is 8 lines missed in both _async_*_await.py that correspond to

Coverage for Python 3.10 (but result is similar for other Python version).

Those lines are missing coverage prior to #732.

Could you provide pointers on what to do? There is the maintenance suggested in #752, is there something else?