rename async keyword in python API

rename async keyword in python API

Bug-Debian: https://bugs.debian.org/912552
Forwarded: https://github.com/potassco/clingo/pull/125

`async` is a reserved keyword starting with python 3.7:
https://docs.python.org/3/whatsnew/3.7.html#summary-release-highlights

The `Control` class method `solve()` now uses the following keyword
argument list:

```
solve(self, assumptions, on_model, on_finish, yield_, async_)

```

closes #125

Gbp-Pq: Name gringo-python37-async.patch