Run staging-installcheck exclusively on python3

It creates invalid reports with python2 and I see no point in debugging
that

gocd/staging.bot.gocd.yaml

@@ -20,11 +20,10 @@ pipelines:
         jobs:
           Run:
             resources:
-            - repo-checker
+            - repo-checker3
             tasks:
             - script: |-
-                cd scripts
-                ./staging-installcheck.py -A https://api.opensuse.org -p openSUSE:Factory
+                python3 ./scripts/staging-installcheck.py -A https://api.opensuse.org -p openSUSE:Factory
   SBot.Regular.Factory:
     group: Factory.pkglistgen
     lock_behavior: unlockWhenFinished
@@ -71,11 +70,10 @@ pipelines:
         jobs:
           Run:
             resources:
-            - repo-checker
+            - repo-checker3
             tasks:
             - script: |-
-                cd scripts
-                ./staging-installcheck.py -A https://api.opensuse.org -p openSUSE:Leap:15.1
+                python3 ./scripts/staging-installcheck.py -A https://api.opensuse.org -p openSUSE:Leap:15.1
   SBot.Regular.Leap.15.1:
     group: Leap.15.1.pkglistgen
     lock_behavior: unlockWhenFinished

gocd/staging.bot.gocd.yaml.erb

@@ -22,11 +22,10 @@ pipelines:
         jobs:
           Run:
             resources:
-            - repo-checker
+            - repo-checker3
             tasks:
             - script: |-
-                cd scripts
-                ./staging-installcheck.py -A https://api.opensuse.org -p openSUSE:<%= project %>
+                python3 ./scripts/staging-installcheck.py -A https://api.opensuse.org -p openSUSE:<%= project %>
   SBot.Regular.<%= nickname %>:
     group: <%= nickname %>.pkglistgen
     lock_behavior: unlockWhenFinished

staging-installcheck.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/python3
 
 from __future__ import print_function
 
@@ -15,6 +15,7 @@ import tempfile
 import osc.core
 import argparse
 import logging
+from urllib.error import HTTPError
 
 from osclib.cache_manager import CacheManager
 from osc import conf
@@ -41,12 +42,6 @@ from osclib.memoize import memoize
 from osclib.util import sha1_short
 from osclib.stagingapi import StagingAPI
 
-try:
-    from urllib.error import HTTPError
-except ImportError:
-    # python 2.x
-    from urllib2 import HTTPError
-
 import ReviewBot
 
 CACHEDIR = CacheManager.directory('repository-meta')