Implement rudimentary asyncio support w/ asyncpg

Using the approach introduced at
https://gist.github.com/zzzeek/6287e28054d3baddc07fa21a7227904e

We can now create asyncio endpoints that are then handled
in "implicit IO" form within the majority of the Core internals.
Then coroutines are re-exposed at the point at which we call
into asyncpg methods.

Patch includes:

* asyncpg dialect

* asyncio package

* engine, result, ORM session classes

* new test fixtures, tests

* some work with pep-484 and a short plugin for the
  pyannotate package, which seems to have so-so results

Change-Id: Idbcc0eff72c4cad572914acdd6f40ddb1aef1a7d
Fixes: #3414

1 files changed, 21 insertions, 0 deletions

diff --git a/lib/sqlalchemy/util/concurrency.py b/lib/sqlalchemy/util/concurrency.py
new file mode 100644
index 000000000..4c4ea20d1
--- /dev/null
+++ b/lib/sqlalchemy/util/concurrency.py
@@ -0,0 +1,21 @@
+from . import compat
+
+
+if compat.py3k:
+    import asyncio
+    from ._concurrency_py3k import await_only
+    from ._concurrency_py3k import await_fallback
+    from ._concurrency_py3k import greenlet
+    from ._concurrency_py3k import greenlet_spawn
+else:
+    asyncio = None
+    greenlet = None
+
+    def await_only(thing):
+        return thing
+
+    def await_fallback(thing):
+        return thing
+
+    def greenlet_spawn(fn, *args, **kw):
+        raise ValueError("Cannot use this function in py2.")