Reland "depot_tools: Bootstrap Python 3 on Linux/Mac"

This is a reland of dbca865e55

Original change's description:
> depot_tools: Bootstrap Python 3 on Linux/Mac
> 
> This will make it possible for developers to execute depot_tools
> scripts using Python 3 in a known environment.
> 
> Bug: 1002153
> Change-Id: I5ff492a49d227c1b5876f49adba020f51a575bdd
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1762664
> Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
> Reviewed-by: Dirk Pranke <dpranke@chromium.org>
> Reviewed-by: Andrii Shyshkalov <tandrii@google.com>

Bug: 1002153
Change-Id: I6938a76ffa05366c21f7c56aed3d07ecb2b84443
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1798845
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

ensure_bootstrap

@@ -9,10 +9,6 @@
 #
 # Unlike "update_depot_tools", this script works with the current
 # checkout, and will not update/sync the "depot_tools" repository.
-#
-# TODO: This duplicates logic in "update_depot_tools". Update that script to
-# invoke this script instead of manually calling "cipd_bin_setup" and other
-# operations.
 
 # Test if this script is running under a MinGW install.  If it is, we will
 # hardcode the paths to Git where possible.
@@ -28,6 +24,11 @@ else
   fi
 fi
 
+if [ "X$DEPOT_TOOLS_BOOTSTRAP_PYTHON3" != "X0" ]; then
+  source "$base_dir/bootstrap_python3"
+  bootstrap_python3
+fi
+
 # Sync CIPD-boostrapped packages.
 source "$base_dir/cipd_bin_setup.sh"
 cipd_bin_setup