extmod/uasyncio: Add Loop.new_event_loop method.

This commit adds Loop.new_event_loop() which is used to reset the singleton
event loop.  This functionality is put here instead of in Loop.close() to
make it possible to write code that is compatible with CPython.

extmod/uasyncio/core.py

@@ -132,13 +132,6 @@ class IOQueue:
 ################################################################################
 # Main run loop
 
-# TaskQueue of Task instances
-_task_queue = TaskQueue()
-
-# Task queue and poller for stream IO
-_io_queue = IOQueue()
-
-
 # Ensure the awaitable is a task
 def _promote_to_task(aw):
     return aw if isinstance(aw, Task) else create_task(aw)
@@ -269,3 +262,16 @@ class Loop:
 # The runq_len and waitq_len arguments are for legacy uasyncio compatibility
 def get_event_loop(runq_len=0, waitq_len=0):
     return Loop
+
+
+def new_event_loop():
+    global _task_queue, _io_queue
+    # TaskQueue of Task instances
+    _task_queue = TaskQueue()
+    # Task queue and poller for stream IO
+    _io_queue = IOQueue()
+    return Loop
+
+
+# Initialise default event loop
+new_event_loop()