uio: add stubs and docs for uio module

Issue: https://github.com/pybricks/support/issues/236

doc/api/index.rst

@@ -79,6 +79,7 @@ findings on our `support page`_ so we can make Pybricks even better.
 
    micropython
    uerrno
+   uio
 
 .. toctree::
    :maxdepth: 1

doc/api/uio.rst

@@ -0,0 +1,6 @@
+:mod:`uio` -- Input/output streams
+==================================
+
+.. note:: This module is not available on the BOOST Move Hub.
+
+.. automodule:: uio

pyproject.toml

@@ -17,6 +17,7 @@ packages = [
     { include = "pybricks", from = "src" },
     { include = "micropython", from = "src" },
     { include = "uerrno", from = "src" },
+    { include = "uio", from = "src" },
 ]
 
 [tool.poetry.dependencies]

src/uio/__init__.py

@@ -0,0 +1,65 @@
+# SPDX-License-Identifier: MIT
+# Copyright (c) 2021 The Pybricks Authors
+#
+# Portions of documentation copied from:
+# https://raw.githubusercontent.com/micropython/micropython/1e6d18c915ccea0b6a19ffec9710d33dd7e5f866/docs/library/uio.rst
+# Copyright (c) 2014-2021, Damien P. George, Paul Sokolovsky, and contributors
+
+"""
+This module contains additional types of ``stream`` (file-like) objects
+and helper functions.
+"""
+
+# TODO: open() is not implemented on Powered Up hubs
+
+from typing import overload
+
+# TODO: MicroPython streams implement '__enter__', '__exit__', 'close', 'read',
+# 'readinto', 'readline', 'write', 'flush', 'getvalue', 'seek', 'tell'
+# and are iterable
+
+
+class BytesIO:
+    @overload
+    def __init__(self) -> None:
+        ...
+
+    @overload
+    def __init__(self, initial_bytes: bytes) -> None:
+        ...
+
+    @overload
+    def __init__(self, alloc_size: int) -> None:
+        ...
+
+    def __init__(self, *args) -> None:
+        """
+        A binary stream using an in-memory bytes buffer.
+
+        Args:
+            initial_bytes: Optional bytes-like object that contains initial data.
+            alloc_size: Optional number of preallocated bytes.
+        """
+
+
+class StringIO:
+    @overload
+    def __init__(self) -> None:
+        ...
+
+    @overload
+    def __init__(self, initial_value: str) -> None:
+        ...
+
+    @overload
+    def __init__(self, alloc_size: int) -> None:
+        ...
+
+    def __init__(self, *args) -> None:
+        """
+        A binary stream using an in-memory string buffer.
+
+        Args:
+            initial_value: Optional string object that contains initial data.
+            alloc_size: Optional number of preallocated bytes.
+        """