sixish: skip mock if unavailable

Since Python 2 doesn't include mock in the stdlib, not everyone has it.
While for unittests we make people install it, for normal hook usage,
we don't want that.  So don't require it to be imported all the time.
When running unittests, it'll still result in a missing failure.

Bug: 28298984
Test: unittests & upload still pass w/out mock installed
Change-Id: Ia9e66775df7d2150537acd0a6383b48d3303b491
diff --git a/rh/sixish.py b/rh/sixish.py
index 81b9630..08f3731 100644
--- a/rh/sixish.py
+++ b/rh/sixish.py
@@ -40,10 +40,14 @@
     import ConfigParser as configparser
 
 
+# We allow mock to be disabled as it's not needed by non-unittest code.
 try:
     import unittest.mock as mock
 except ImportError:
-    import mock
+    try:
+        import mock
+    except ImportError:
+        pass
 
 
 if sys.version_info.major < 3: