Revert "depot_tools: Run using Python 3 by default."

This reverts commit 01ed3589.

Reason for revert: Broke tools on Windows with bash

Original change's description:
> depot_tools: Run using Python 3 by default.
> 
> Run gclient, roll-dep, fetch and custom git commands (i.e. git-cl, git-rebase-update, git-new-branch, etc.)
> using vpython3 by default.
> 
> Change-Id: I4eecddafa6ca4c5f82ec097615c79d2a741613e7
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2113550
> Reviewed-by: Anthony Polito <apolito@google.com>
> Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

TBR=ehmaldonado@chromium.org,ajp@chromium.org,apolito@google.com,infra-scoped@luci-project-accounts.iam.gserviceaccount.com,sokcevic@google.com

Change-Id: If29ad72feaa215b8b282f2f6c9911700dd2f228c
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2116728Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Peter Kasting <pkasting@chromium.org>

fetch

@@ -16,6 +16,6 @@ elif [[ $GCLIENT_PY3 == 0 ]]; then
   # Explicitly run on Python 2
   PYTHONDONTWRITEBYTECODE=1 exec vpython "$base_dir/fetch.py" "$@"
 else
-  # Run on Python 3, allows default to be flipped.
-  PYTHONDONTWRITEBYTECODE=1 exec vpython3 "$base_dir/fetch.py" "$@"
+  # Run on Python 2 for now, allows default to be flipped.
+  PYTHONDONTWRITEBYTECODE=1 exec vpython "$base_dir/fetch.py" "$@"
 fi

gclient

@@ -33,6 +33,6 @@ elif [[ $GCLIENT_PY3 == 0 ]]; then
   # Explicitly run on Python 2
   PYTHONDONTWRITEBYTECODE=1 exec vpython "$base_dir/gclient.py" "$@"
 else
-  # Run on Python 3, allows default to be flipped.
-  PYTHONDONTWRITEBYTECODE=1 exec vpython3 "$base_dir/gclient.py" "$@"
+  # Run on Python 2 for now, allows default to be flipped.
+  PYTHONDONTWRITEBYTECODE=1 exec vpython "$base_dir/gclient.py" "$@"
 fi

python_runner.sh

@@ -59,6 +59,6 @@ elif [[ $GCLIENT_PY3 = 0 ]]; then
   # Explicitly run on Python 2
   vpython "$DEPOT_TOOLS/$SCRIPT" "$@"
 else
-  # Run on Python 3, allows default to be flipped.
-  vpython3 "$DEPOT_TOOLS/$SCRIPT" "$@"
+  # Run on Python 2 for now, allows default to be flipped.
+  vpython "$DEPOT_TOOLS/$SCRIPT" "$@"
 fi

roll-dep

@@ -16,6 +16,6 @@ elif [[ $GCLIENT_PY3 = 0 ]]; then
   # Explicitly run on Python 2
   PYTHONDONTWRITEBYTECODE=1 exec vpython "$base_dir/roll_dep.py" "$@"
 else
-  # Run on Python 3, allows default to be flipped.
-  PYTHONDONTWRITEBYTECODE=1 exec vpython3 "$base_dir/roll_dep.py" "$@"
+  # Run on Python 2 for now, allows default to be flipped.
+  PYTHONDONTWRITEBYTECODE=1 exec vpython "$base_dir/roll_dep.py" "$@"
 fi

tests/gclient_smoketest.py

@@ -40,7 +40,6 @@ class GClientSmokeBase(fake_repos.FakeReposTestBase):
     self.env['DEPOT_TOOLS_METRICS'] = '0'
     # Suppress Python 3 warnings and other test undesirables.
     self.env['GCLIENT_TEST'] = '1'
-    self.env['GCLIENT_PY3'] = '0' if sys.version_info.major == 2 else '1'
     self.maxDiff = None
 
   def gclient(self, cmd, cwd=None, error_ok=False):