Add i18n support

There are translatable strings in this library and we need to
support that properly. Replace the stub _() with a proper
implementation based on oslo.i18n.

oslo_upgradecheck/upgradecheck.py

@@ -21,9 +21,7 @@ import enum
 from oslo_config import cfg
 import prettytable
 
-
-def _(s):
-    return s
+from oslo_upgradecheck._i18n import _
 
 
 class UpgradeCheckCode(enum.IntEnum):

requirements.txt

@@ -4,5 +4,6 @@
 
 Babel>=1.3
 oslo.config
+oslo.i18n
 enum34
 prettytable

tox.ini

@@ -58,3 +58,6 @@ show-source = True
 ignore = E123,E125
 builtins = _
 exclude=.venv,.git,.tox,dist,doc,*lib/python*,*egg,build
+
+[hacking]
+import_exception = oslo_upgradecheck._i18n