Add option to suppress syntax warnings in checkdeps.py

Executing `vpython3 testing/scripts/checkdeps.py` command for a thorough
test prints out many lines of SyntaxWarning such as `<string>:21:
SyntaxWarning: invalid escape sequence '\.'`, and almost all of them are
irrelevant to local changes.

Omitting these prints drastically reduces the stdout size and is
particularly beneficial if this command is used in an agentic coding
workflow in terms of LLM context management.

Change-Id: I1fc18dd2881db9d9a39135b1048e3b1b9691d14c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7698740
Reviewed-by: Junji Watanabe <jwata@google.com>
Auto-Submit: Keybo Qian <keybo@google.com>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Matt Stark <msta@google.com>
Commit-Queue: Junji Watanabe <jwata@google.com>
Cr-Commit-Position: refs/heads/main@{#1605236}
NOKEYCHECK=True
GitOrigin-RevId: 09ceea731c94a5b5f0ef626eb361fba85fd7d256