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/asyncio/threads.py

@@ -0,0 +1,21 @@
+"""High-level support for working with threads in asyncio"""
+
+import functools
+
+from . import events
+
+
+__all__ = "to_thread",
+
+
+async def to_thread(func, /, *args, **kwargs):
+    """Asynchronously run function *func* in a separate thread.
+
+    Any *args and **kwargs supplied for this function are directly passed
+    to *func*.
+
+    Return an asyncio.Future which represents the eventual result of *func*.
+    """
+    loop = events.get_running_loop()
+    func_call = functools.partial(func, *args, **kwargs)
+    return await loop.run_in_executor(None, func_call)