Improvement
- Added the `AsyncClient` wrapper which is intended for `asyncio` environment usage. `AsyncClient` has the same methods
with the same parameters as the standard `Client`, but they are coroutines when applicable. Internally, these methods
from the `Client` that perform I/O operations are wrapped in a
[run_in_executor](https://docs.python.org/3/library/asyncio-eventloop.html#asyncio.loop.run_in_executor) call.
See also the updated [run_async example](./examples/run_async.py).