Catch and ignore ImportError on skylab_inventory for unittests.

BUG=chromium:826492
TEST=run unittests without skylab_inventory in site-packages

Change-Id: I4c89052593bc0429dd9bf7a89f70e06e2d64270c
Reviewed-on: https://chromium-review.googlesource.com/1033971
Commit-Ready: Ningning Xia <[email protected]>
Tested-by: Ningning Xia <[email protected]>
Reviewed-by: Ningning Xia <[email protected]>
diff --git a/cli/skylab_utils.py b/cli/skylab_utils.py
index 6533085..df8e04b 100644
--- a/cli/skylab_utils.py
+++ b/cli/skylab_utils.py
@@ -11,7 +11,11 @@
 
 from autotest_lib.client.common_lib import revision_control
 from chromite.lib import gob_util
-from skylab_inventory import text_manager
+
+try:
+    from skylab_inventory import text_manager
+except ImportError:
+    pass
 
 
 INTERNAL_GERRIT_HOST = 'chrome-internal-review.googlesource.com'
@@ -24,6 +28,10 @@
         'infra_internal/skylab_inventory/\\+/([0-9]*)')
 
 
+class SkylabInventoryNotImported(Exception):
+    """skylab_inventory is not imported."""
+
+
 class InventoryRepoChangeNotFound(Exception):
     """Error raised when no inventory repo change number is found."""