bpo-32309: Implement asyncio.to_thread() (GH-20143)

Implements `asyncio.to_thread`, a coroutine for asynchronously running IO-bound functions in a separate thread without blocking the event loop. See the discussion starting from [here](https://github.com/python/cpython/pull/18410GH-issuecomment-628930973) in GH-18410 for context.

Automerge-Triggered-By: @aeros
(cherry picked from commit cc2bbc2227)

Co-authored-by: Kyle Stanley <aeros167@gmail.com>

Lib/test/test_asyncio/test_threads.py

@@ -0,0 +1,79 @@
+"""Tests for asyncio/threads.py"""
+
+import asyncio
+import unittest
+
+from unittest import mock
+from test.test_asyncio import utils as test_utils
+
+
+def tearDownModule():
+    asyncio.set_event_loop_policy(None)
+
+
+class ToThreadTests(test_utils.TestCase):
+    def setUp(self):
+        super().setUp()
+        self.loop = asyncio.new_event_loop()
+        asyncio.set_event_loop(self.loop)
+
+    def tearDown(self):
+        self.loop.run_until_complete(
+            self.loop.shutdown_default_executor())
+        self.loop.close()
+        asyncio.set_event_loop(None)
+        self.loop = None
+        super().tearDown()
+
+    def test_to_thread(self):
+        async def main():
+            return await asyncio.to_thread(sum, [40, 2])
+
+        result = self.loop.run_until_complete(main())
+        self.assertEqual(result, 42)
+
+    def test_to_thread_exception(self):
+        def raise_runtime():
+            raise RuntimeError("test")
+
+        async def main():
+            await asyncio.to_thread(raise_runtime)
+
+        with self.assertRaisesRegex(RuntimeError, "test"):
+            self.loop.run_until_complete(main())
+
+    def test_to_thread_once(self):
+        func = mock.Mock()
+
+        async def main():
+            await asyncio.to_thread(func)
+
+        self.loop.run_until_complete(main())
+        func.assert_called_once()
+
+    def test_to_thread_concurrent(self):
+        func = mock.Mock()
+
+        async def main():
+            futs = []
+            for _ in range(10):
+                fut = asyncio.to_thread(func)
+                futs.append(fut)
+            await asyncio.gather(*futs)
+
+        self.loop.run_until_complete(main())
+        self.assertEqual(func.call_count, 10)
+
+    def test_to_thread_args_kwargs(self):
+        # Unlike run_in_executor(), to_thread() should directly accept kwargs.
+        func = mock.Mock()
+
+        async def main():
+            await asyncio.to_thread(func, 'test', something=True)
+
+        self.loop.run_until_complete(main())
+        func.assert_called_once_with('test', something=True)
+
+
+if __name__ == "__main__":
+    unittest.main()