Reland "Force Python 3 to be used in build."

This reverts commit b6172196.

Reason for revert: We rolled GN to a new version with a fix for
the win dev issues, so hopefully is good to go now.

Original change's description:
> Revert "Force Python 3 to be used in build."
>
> This reverts commit bb97ab9b.
>
> Reason for revert: Various breakages.
>
> Original change's description:
> > Force Python 3 to be used in build.
> >
> > This CL makes Python 3 be the default script executable used by Ninja
> > to run action() and action_foreach()'s during the build.
> >
> > This does *not* mean that everything works with Python3; many targets
> > may still use Python 2 via the python2_action() and
> > python2_action_foreach() targets or via custom logic.
> >
> > However, it does mean that it should prevent regressions from
> > creeping in.
> >
> > This CL should not be taken to imply that we can start to write
> > Python3-specific code that can run during the build; we should
> > still be trying to ensure that everything is compatible with Python2.
> >
> > Also, this CL does not enable the use of vpython in actions; we should
> > still avoid that because vpython is slower than direct Python
> > invocations.
> >
> > Bug: 1112471
> > Change-Id: I472b032b500715156d955b0758640a7ed0384305
> > Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2515787
> > Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
> > Commit-Queue: Dirk Pranke <dpranke@google.com>
> > Cr-Commit-Position: refs/heads/master@{#836234}
>
> TBR=dpranke@google.com,brucedawson@chromium.org,chromium-scoped@luci-project-accounts.iam.gserviceaccount.com
>
> Change-Id: I72039140f401f1fefc1b515dcfe34f1984b0d914
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Bug: 1112471
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2587576
> Reviewed-by: Dirk Pranke <dpranke@google.com>
> Commit-Queue: Dirk Pranke <dpranke@google.com>
> Cr-Commit-Position: refs/heads/master@{#836284}

TBR=dpranke@google.com,brucedawson@chromium.org,tikuta@chromium.org,chromium-scoped@luci-project-accounts.iam.gserviceaccount.com

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 1112471
Change-Id: I55ee8ccb381b5aeeac58a85a6168765ed40457ba
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2630725Reviewed-by: Dirk Pranke <dpranke@google.com>
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@google.com>
Cr-Commit-Position: refs/heads/master@{#844885}

.gn

@@ -8,6 +8,10 @@ import("//third_party/angle/dotfile_settings.gni")
 # The location of the build configuration file.
 buildconfig = "//build/config/BUILDCONFIG.gn"
 
+# The python interpreter to use by default. On Windows, this will look
+# for python3.exe and python3.bat.
+script_executable = "python3"
+
 # These arguments override the default values for items in a declare_args
 # block. "gn args" in turn can override these.
 #