autoninja: ignore ResourceWarning from subprocess for ninjalog_uploader.py

The main process exits right after ninja_uploader.py subprocess starts,
which may print ResourceWarning from subprocess.
https://github.com/python/cpython/blob/e319f774f9e766a2b92949444a2d46081df3363a/Lib/subprocess.py#L946-L947
This CL ignores the warning message.

Change-Id: I800387194763e60a16437b3bd41fa282b605dda7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5782795
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Auto-Submit: Junji Watanabe <jwata@google.com>
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>

autoninja.py

@@ -406,6 +406,7 @@ def _main_inner(input_args, build_id, should_collect_logs=False):
 
 
 def _upload_ninjalog(args):
+    warnings.simplefilter("ignore", ResourceWarning)
     # Run upload script without wait.
     creationflags = 0
     if platform.system() == "Windows":