kenel_config: Fix error in _read_config

readlines() won't return an object with __exit__ interface.
Fix this.

BUG=None
TEST=None

Change-Id: I0d34cdef12a383f5ee46b5a3c604033fa6a4d92e
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/autotest/+/2498107
Tested-by: Po-Hsien Wang <pwang@chromium.org>
Auto-Submit: Po-Hsien Wang <pwang@chromium.org>
Reviewed-by: Derek Beckett <dbeckett@chromium.org>
Commit-Queue: Derek Beckett <dbeckett@chromium.org>
diff --git a/client/cros/kernel_config.py b/client/cros/kernel_config.py
index 6022c86..5a9fa2e 100644
--- a/client/cros/kernel_config.py
+++ b/client/cros/kernel_config.py
@@ -145,7 +145,7 @@
         if not os.path.exists(filename):
             utils.system("modprobe configs", ignore_status=True)
         if os.path.exists(filename):
-            with gzip.open(filename, "r").readlines() as rf:
+            with gzip.open(filename, "r") as rf:
                 return rf.readlines()
 
         filename = '/boot/config-%s' % utils.system_output('uname -r')