tests/cpydiff: Document lack of OSError errno subtype mapping.

Signed-off-by: Anson Mansfield <amansfield@mantaro.com>

tests/cpydiff/types_oserror_errnomap.py

@@ -0,0 +1,48 @@
+"""
+categories: Types,OSError
+description: OSError constructor returns a plain OSError for all errno values, rather than a relevant subtype.
+cause: MicroPython does not include the CPython-standard OSError subclasses.
+workaround: Catch OSError and use its errno attribute to discriminate the cause.
+"""
+
+import errno
+
+errno_list = [  # i.e. the set implemented by micropython
+    errno.EPERM,
+    errno.ENOENT,
+    errno.EIO,
+    errno.EBADF,
+    errno.EAGAIN,
+    errno.ENOMEM,
+    errno.EACCES,
+    errno.EEXIST,
+    errno.ENODEV,
+    errno.EISDIR,
+    errno.EINVAL,
+    errno.EOPNOTSUPP,
+    errno.EADDRINUSE,
+    errno.ECONNABORTED,
+    errno.ECONNRESET,
+    errno.ENOBUFS,
+    errno.ENOTCONN,
+    errno.ETIMEDOUT,
+    errno.ECONNREFUSED,
+    errno.EHOSTUNREACH,
+    errno.EALREADY,
+    errno.EINPROGRESS,
+]
+
+
+def errno_output_type(n):
+    try:
+        raise OSError(n, "")
+    except OSError as e:
+        return f"{type(e).__name__}"
+    except Exception as e:
+        return f"non-OSError {type(e).__name__}"
+    else:
+        return "no error"
+
+
+for n in errno_list:
+    print(errno.errorcode[n], "=", errno_output_type(n))