extmod/modselect: Handle pending events before entering poll.

This allows tests like `asyncio_event_queue.py` to succeed under the native
emitter when poll is enabled.

Signed-off-by: Jeff Epler <jepler@unpythonic.net>

docs/library/micropython.rst

@@ -132,7 +132,8 @@ Functions
      handler (an IRQ).
    - Inside native code functions, scheduled functions are not called unless
      the native code calls a function that specifically does so.
-   - ``time.sleep`` and ``time.sleep_ms``, including zero-duration sleeps,
+   - Certain functions including ``poll.poll``, ``poll.ipoll``,
+     ``time.sleep`` and ``time.sleep_ms`` (including zero-duration sleeps)
      will call scheduled functions.
 
    A use for this function is to schedule a callback from a preempting IRQ.

docs/library/select.rst

@@ -76,6 +76,9 @@ Methods
 
    In case of timeout, an empty list is returned.
 
+   Calling ``poll.poll`` is guaranteed to call pending callback functions
+   before entering the polling loop.
+
    .. admonition:: Difference to CPython
       :class: attention
 
@@ -93,6 +96,9 @@ Methods
    won't be processed until new mask is set with `poll.modify()`. This
    behaviour is useful for asynchronous I/O schedulers.
 
+   Calling ``poll.ipoll`` is guaranteed to call pending callback functions
+   before entering the polling loop.
+
    .. admonition:: Difference to CPython
       :class: attention
 

extmod/modselect.c

@@ -342,6 +342,8 @@ static mp_uint_t poll_set_poll_until_ready_or_timeout(poll_set_t *poll_set, size
     mp_uint_t start_ticks = mp_hal_ticks_ms();
     bool has_timeout = timeout != (mp_uint_t)-1;
 
+    mp_handle_pending(true);
+
     #if MICROPY_PY_SELECT_POSIX_OPTIMISATIONS
 
     for (;;) {