pw_hdlc: Handle sys.stdout without a buffer attribute

The buffer attribute is not required in TextIOBase instances. Support
wrapped versions of sys.stdout that do not offer it by deferring to
sys.__stdout__.buffer.

Change-Id: I9677da0cb16e889562f1d2737d3b275f16ac97ad
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/154496
Presubmit-Verified: CQ Bot Account <pigweed-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Armando Montanez <amontanez@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
Pigweed-Auto-Submit: Wyatt Hepler <hepler@google.com>

pw_hdlc/py/pw_hdlc/rpc.py

@@ -133,7 +133,14 @@ def run(self) -> NoReturn:
                     executor.submit(self._frame_handler, frame)
 
 
-def write_to_file(data: bytes, output: BinaryIO = sys.stdout.buffer):
+# Writes to stdout by default, but sys.stdout.buffer is not guaranteed to exist
+# (see https://docs.python.org/3/library/io.html#io.TextIOBase.buffer). Defer
+# to sys.__stdout__.buffer if sys.stdout is wrapped with something that does not
+# offer it.
+def write_to_file(
+    data: bytes,
+    output: BinaryIO = getattr(sys.stdout, 'buffer', sys.__stdout__.buffer),
+) -> None:
     output.write(data + b'\n')
     output.flush()