locking: fix under Python 3

os.pipe returns fd's that are opened in binary mode.
The unittests pass in general, but lib.parallel doesn't work fully
yet, so we can't enable it.

TEST=`./run_tests` passes

Change-Id: I66d67a6f1ce4d3197513472f1038bc0d8b88ee97
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/1906753
Reviewed-by: Jack Rosenthal <jrosenth@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
diff --git a/lib/locking.py b/lib/locking.py
index 38f65df..f527a9f 100644
--- a/lib/locking.py
+++ b/lib/locking.py
@@ -357,7 +357,7 @@
     return os.read(self.read_fd, size)
-  def Post(self, data='!'):
+  def Post(self, data=b'!'):
     """Write |data| to the pipe.