docs: Hide python warnings when generating docs

Those warnings end up going into man pages, which is not desired. The
warning says that pipes are deprecated, and will be removed in py3.13.

R=yiwzhang@google.com

Bug: 396174247
Change-Id: Ieb7f36de944e8b7b5e68f78c87a61b04e663f9d1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6268243
Reviewed-by: Yiwei Zhang <yiwzhang@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@chromium.org>

man/src/make_docs.sh

@@ -226,7 +226,7 @@ fi
 (
   export GIT_DIR="$(git rev-parse --git-dir)" &&
   cd git/Documentation &&
-  GIT_EDITOR=true make "${MAN1_TARGETS[@]}" "${MAN7_TARGETS[@]}" "${HTML_TARGETS[@]}"
+  GIT_EDITOR=true PYTHONWARNINGS="ignore" make "${MAN1_TARGETS[@]}" "${MAN7_TARGETS[@]}" "${HTML_TARGETS[@]}"
 )
 
 for x in "${HTML_TARGETS[@]}"