extmod/uasyncio: Add asyncio.current_task().

Matches CPython behavior.

Fixes #6686

docs/library/uasyncio.rst

@@ -40,6 +40,10 @@ Core functions
 
     Returns the corresponding `Task` object.
 
+.. function:: current_task()
+
+    Return the `Task` object associated with the currently running task.
+
 .. function:: run(coro)
 
     Create a new task from the given coroutine and run it until it completes.

extmod/uasyncio/core.py

@@ -264,6 +264,10 @@ def get_event_loop(runq_len=0, waitq_len=0):
     return Loop
 
 
+def current_task():
+    return cur_task
+
+
 def new_event_loop():
     global _task_queue, _io_queue
     # TaskQueue of Task instances

tests/extmod/uasyncio_current_task.py

@@ -0,0 +1,25 @@
+# Test current_task() function
+
+try:
+    import uasyncio as asyncio
+except ImportError:
+    try:
+        import asyncio
+    except ImportError:
+        print("SKIP")
+        raise SystemExit
+
+
+async def task(result):
+    result[0] = asyncio.current_task()
+
+
+async def main():
+    result = [None]
+    t = asyncio.create_task(task(result))
+    await asyncio.sleep(0)
+    await asyncio.sleep(0)
+    print(t is result[0])
+
+
+asyncio.run(main())

tests/extmod/uasyncio_current_task.py.exp

@@ -0,0 +1 @@
+True