findmissing: Add 5.7 as stable release, plus other pylint mandated changes

v5.7.y is the most recent stable release.

Other unrelated changes, necessary for repo upload to pass:

- import MySQLdb now results in import-error. This is pointless, since
  we make MySQLdb available (only) in a virtual environment. We could of
  course install MySQLdb to make pylint happy, but that would be even
  more pointless. Add "pylint: disable=import-error" to disable the check.
- pylint now complains about "class KernelMetadata(object)" and
  states that "object" is unnecessary. Let's hope that this is true
  and remove "object" from the class definition.

BUG=None
TEST=Run geopen tool

Change-Id: I63ebf8cdf55ebe966c7ea58e27e2ec53de35ad99
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/2234285
Reviewed-by: Curtis Malainey <cujomalainey@chromium.org>
Commit-Queue: Guenter Roeck <groeck@chromium.org>
Tested-by: Guenter Roeck <groeck@chromium.org>
diff --git a/contrib/findmissing/common.py b/contrib/findmissing/common.py
index cddf1bb..45592da 100755
--- a/contrib/findmissing/common.py
+++ b/contrib/findmissing/common.py
@@ -14,7 +14,7 @@
 import time
 from enum import Enum
 import subprocess
-import MySQLdb
+import MySQLdb # pylint: disable=import-error
 
 import initdb_upstream
 import initdb_stable
@@ -33,7 +33,7 @@
 
 # Order BRANCHES from oldest to newest
 CHROMEOS_BRANCHES = ['4.4', '4.14', '4.19', '5.4']
-STABLE_BRANCHES = ['4.4', '4.9', '4.14', '4.19', '5.4', '5.6']
+STABLE_BRANCHES = ['4.4', '4.9', '4.14', '4.19', '5.4', '5.6', '5.7']
 
 UPSTREAM_START_BRANCH = 'v%s' % CHROMEOS_BRANCHES[0]
 
@@ -64,7 +64,7 @@
     linux_upstream = 4
 
 
-class KernelMetadata(object):
+class KernelMetadata():
     """Object to group kernel Metadata."""
     path = None
     repo = None