extmod/uasyncio: Add StreamReader/StreamWriter as aliases of Stream cls.

To be compatible with CPython.  Fixes issue #5847.

docs/library/uasyncio.rst

@@ -177,7 +177,8 @@ TCP stream connections
 .. class:: Stream()
     
     This represents a TCP stream connection.  To minimise code this class implements
-    both a reader and a writer.
+    both a reader and a writer, and both ``StreamReader`` and ``StreamWriter`` alias to
+    this class.
 
 .. method:: Stream.get_extra_info(v)
 

extmod/uasyncio/__init__.py

@@ -12,6 +12,8 @@ _attrs = {
     "Lock": "lock",
     "open_connection": "stream",
     "start_server": "stream",
+    "StreamReader": "stream",
+    "StreamWriter": "stream",
 }
 
 # Lazy loader, effectively does:

extmod/uasyncio/stream.py

@@ -53,6 +53,11 @@ class Stream:
         self.out_buf = b""
 
 
+# Stream can be used for both reading and writing to save code size
+StreamReader = Stream
+StreamWriter = Stream
+
+
 # Create a TCP stream connection to a remote host
 async def open_connection(host, port):
     from uerrno import EINPROGRESS