pre-upload: don't echo black check command line

Don't echo the command line used to run the black check on python files.
This follows the convention of all other checks to only print out
warnings and errors.

BUG=none
TEST=Run the pre-upload unit test

Change-Id: I5617aa891b58dbd4206e3e30e7f2246dc6b1684c
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/repohooks/+/3999486
Reviewed-by: Alex Klein <saklein@chromium.org>
Tested-by: Keith Short <keithshort@chromium.org>
Commit-Queue: Keith Short <keithshort@chromium.org>

pre-upload.py

diff --git a/pre-upload.py b/pre-upload.py
index a54617a..dd06493 100755
--- a/pre-upload.py
+++ b/pre-upload.py
@@ -2360,6 +2360,7 @@
                 f"--stdin-filename={file}",
                 "-",
             ],
+            print_cmd=False,
             input=contents,
             check=False,
             stdout=subprocess.PIPE,