Add support for __await__, __aiter__, and __anext__ protocols (#1842)

tests/test_async.py

@@ -0,0 +1,23 @@
+import asyncio
+import pytest
+from pybind11_tests import async_module as m
+
+
+@pytest.fixture
+def event_loop():
+    loop = asyncio.new_event_loop()
+    yield loop
+    loop.close()
+
+
+async def get_await_result(x):
+    return await x
+
+
+def test_await(event_loop):
+    assert 5 == event_loop.run_until_complete(get_await_result(m.SupportsAsync()))
+
+
+def test_await_missing(event_loop):
+    with pytest.raises(TypeError):
+        event_loop.run_until_complete(get_await_result(m.DoesNotSupportAsync()))