power_BatteryCharge: pin to Python2

Adding shebang to use Python2 by default.

BUG=b:135986738
TEST=test_that -b kukui ${DUT_IP} \
--autotest_dir=${AUTOTEST_DIR}/files \
f:.*suite_HWQual/control.battery_charge_time

Change-Id: Ia9f64a7cec8ad6fd0b54d0b9c6c1a8b5b5cf1e03
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/autotest/+/1978126
Tested-by: Fei Shao <fshao@chromium.org>
Reviewed-by: Mengqi Guo <mqg@chromium.org>
Commit-Queue: Fei Shao <fshao@chromium.org>
diff --git a/client/site_tests/power_BatteryCharge/power_BatteryCharge.py b/client/site_tests/power_BatteryCharge/power_BatteryCharge.py
index 7026751..35dfdfc 100755
--- a/client/site_tests/power_BatteryCharge/power_BatteryCharge.py
+++ b/client/site_tests/power_BatteryCharge/power_BatteryCharge.py
@@ -1,3 +1,4 @@
+#!/usr/bin/python2
 # Copyright (c) 2010 The Chromium OS Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.