Python: Fix some more async parsing problems

Turns out we were not setting the `is_async` field on anything except
`async for` statements. This commit makes it so that we also do this for
`async def` and `async with`, and adds a test that this produces the
same behaviour as the old parser.

python/extractor/tests/parser/async-await.py

@@ -0,0 +1,8 @@
+async def foo():
+    await bar() + await baz()
+
+    async with foo() as bar, baz() as quux:
+        pass
+
+    async for spam in eggs:
+        pass