py3: stop using deprecated file open mode
py3: stop using deprecated file open mode

R=iannucci, yuanjunh

Change-Id: Ic7a97a7497ca7831c0868f60ab5013acec7c4998
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/recipes-py/+/3105724
Commit-Queue: Yuanjun Huang <a5f4b8445ce5e958d19c0a7caeadcd07c9d18770@google.com>
Auto-Submit: Yiwei Zhang <50b2a565e2e78c292794832469a30ce5abc9959c@google.com>
Reviewed-by: Yuanjun Huang <a5f4b8445ce5e958d19c0a7caeadcd07c9d18770@google.com>

luci/recipes-py,luci/recipes-py