Revert "autoninja: roll back to python from python3"

This reverts commit 3253a1de.

Reason for revert:
Blocker is fixed in https://crbug.com/1168365#c8

Original change's description:
> autoninja: roll back to python from python3
>
> python3 is currently not recognized on git bash.
> https://crrev.com/c/2629087/15#message-b5093e69201df42adb7ac643e4f97f9f366a299d
>
> This is partial revert of
> https://crrev.com/c/2629087
> https://crrev.com/c/2631396
>
> Bug: b/177465438
> Change-Id: I1b2698f5fc27144035598249c43800d25801a0f7
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2638834
> Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
> Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
> Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
> Auto-Submit: Takuto Ikuta <tikuta@chromium.org>

Bug: b/177465438, 1168365
Change-Id: I368e28aeec9c585a96c8b44f030609eac58698f2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2752772Reviewed-by: Dirk Pranke <dpranke@google.com>
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>

autoninja

@@ -5,7 +5,7 @@
 # found in the LICENSE file.
 
 # Set unique build ID.
-AUTONINJA_BUILD_ID="$(python -c "import uuid; print(uuid.uuid4())")"
+AUTONINJA_BUILD_ID="$(python3 -c "import uuid; print(uuid.uuid4())")"
 export AUTONINJA_BUILD_ID
 
 if [ "$NINJA_SUMMARIZE_BUILD" == "1" ]; then
@@ -21,16 +21,16 @@ if [ "$NINJA_SUMMARIZE_BUILD" == "1" ]; then
 fi
 if eval "$command"; then
   if [ "$NINJA_SUMMARIZE_BUILD" == "1" ]; then
-    python "$(dirname -- "$0")/post_build_ninja_summary.py" "$@"
+    python3 "$(dirname -- "$0")/post_build_ninja_summary.py" "$@"
   fi
 
   # Collect ninjalog from googler.
-  python "$(dirname -- "$0")/ninjalog_uploader_wrapper.py" --cmd $command
+  python3 "$(dirname -- "$0")/ninjalog_uploader_wrapper.py" --cmd $command
   exit
 fi
 
 # Collect ninjalog from googler.
-python "$(dirname -- "$0")/ninjalog_uploader_wrapper.py" --cmd $command
+python3 "$(dirname -- "$0")/ninjalog_uploader_wrapper.py" --cmd $command
 
 # Return an error code of 1 so that if a developer types:
 # "autoninja chrome && chrome" then chrome won't run if the build fails.