uerrno: add uerrno module stubs and docs

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

src/uerrno/__init__.py

@@ -0,0 +1,69 @@
+# SPDX-License-Identifier: MIT
+# Copyright (c) 2021 The Pybricks Authors
+#
+# Portions of documentation copied from:
+# https://raw.githubusercontent.com/micropython/micropython/1e6d18c915ccea0b6a19ffec9710d33dd7e5f866/docs/library/uerrno.rst
+# Copyright (c) 2014-2021, Damien P. George, Paul Sokolovsky, and contributors
+
+"""
+This module provides access to symbolic error codes for `OSError` exception.
+"""
+
+from typing import Dict
+
+EAGAIN: int
+"""
+The operation is not complete and should be tried again soon.
+"""
+
+EBUSY: int
+"""
+The device or resource is busy and cannot be used right now.
+"""
+
+ECANCELED: int
+"""
+The operation was canceled.
+"""
+
+EINVAL: int
+"""
+An invalid argument was given. Usually ``ValueError`` is used instead.
+"""
+
+EIO: int
+"""
+An unspecified error occurred.
+"""
+
+ENODEV: int
+"""
+Device was not found. For example, a sensor or motor is not plugged in the correct port.
+"""
+
+EOPNOTSUPP: int
+"""
+The operation is not supported on this hub or on the connected device.
+"""
+
+EPERM: int
+"""
+The operation cannot be performed in the current state.
+"""
+
+ETIMEDOUT: int
+"""
+The operation timed out.
+"""
+
+# TODO: ev3dev has additional constants
+# https://github.com/pybricks/pybricks-micropython/blob/11f19bc9c24fde66aa8ad42233a345e6683f5beb/bricks/ev3dev/mpconfigport.h#L156-L203
+
+errorcode: Dict[int, str]
+"""
+Dictionary mapping numeric error codes to strings with symbolic error
+code (see above)::
+
+    >>> print(uerrno.errorcode[uerrno.EEXIST])
+    EEXIST
+"""