extmod/uasyncio: Add StreamReader.readexactly(n) method.

It raises on EOFError instead of an IncompleteReadError (which is what
CPython does).  But the latter is derived from EOFError so code compatible
with MicroPython and CPython can be written by catching EOFError (eg see
included test).

Fixes issue #6156.

Signed-off-by: Damien George <damien@micropython.org>

extmod/uasyncio/stream.py

@@ -30,6 +30,18 @@ class Stream:
         yield core._io_queue.queue_read(self.s)
         return self.s.read(n)
 
+    async def readexactly(self, n):
+        r = b""
+        while n:
+            yield core._io_queue.queue_read(self.s)
+            r2 = self.s.read(n)
+            if r2 is not None:
+                if not len(r2):
+                    raise EOFError
+                r += r2
+                n -= len(r2)
+        return r
+
     async def readline(self):
         l = b""
         while True: