Add support for python 3.6

[maffoo]

We'd like to be able to use duet in python3.6 since it's still used by colab and internally at google (see quantumlib/Cirq#4009). Among other things, this will require adding back in support for AsyncContextManager and adding a 3.6-only dependency on contextvars. There may be some other changes required as well.

[fedimser]

I found other thing which duet uses and which is not available in Python 3.6: postponed evalutaion of annotations (PEP 563). This was added in Python 3.7, so in Python 3.6 statement from __future__ import annotations result in an error.

I am thinking about dropping this, i.e. removing all from __future__ import annotations lines. If I understand correctly how typing works, we just will have to quote type annotations in few places.