[openwrt/openwrt] build: fix compatibility with python 3.6

[LEDE Commits]

ynezz pushed a commit to openwrt/openwrt.git, branch master:
https://git.openwrt.org/3caad5150c2011a7dac462acaa06d0e69f8ed005

commit 3caad5150c2011a7dac462acaa06d0e69f8ed005
Author: Huangbin Zhan <zhanhb88 at gmail.com>
AuthorDate: Tue Jul 7 07:14:42 2020 +0800

    build: fix compatibility with python 3.6
    
    On a system python3 is linked to python3.6, fail to perform json_overview_image_info
     and got `TypeError: __init__() got an unexpected keyword argument 'capture_output'`.
    This patch emulate the behaviour on python 3.7+.
    
    Signed-off-by: Huangbin Zhan <zhanhb88 at gmail.com>
---
 scripts/json_overview_image_info.py | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/scripts/json_overview_image_info.py b/scripts/json_overview_image_info.py
index 8f3525f890..c76a442840 100755
--- a/scripts/json_overview_image_info.py
+++ b/scripts/json_overview_image_info.py
@@ -2,7 +2,7 @@
 
 from os import getenv, environ
 from pathlib import Path
-from subprocess import run
+from subprocess import run, PIPE
 from sys import argv
 import json
 
@@ -42,10 +42,11 @@ if output:
             "val.DEFAULT_PACKAGES",
             "val.ARCH_PACKAGES",
         ],
-        capture_output=True,
+        stdout=PIPE,
+        stderr=PIPE,
         check=True,
         env=environ.copy().update({"TOPDIR": Path().cwd()}),
-        text=True,
+        universal_newlines=True,
     ).stdout.splitlines()
 
     output["default_packages"] = default_packages.split()