Remove airship-tempest-plugin from blacklist

This commit removes airship-tempest-plugin entry from the BLACKLIST
since the patch was already merged. And airship/tempest-plugin has a
gate job to verify it, too. There is no reason to store it in the
BLACKLIST anymore.

And this commit also make plugin-sanity-check's basepython python3
because we should use it as a default.

Change-Id: I9c50d327df65fecf8510f6f54f06e9f42da9bea0
diff --git a/tox.ini b/tox.ini
index 230249f..9bee3dd 100644
--- a/tox.ini
+++ b/tox.ini
@@ -265,6 +265,7 @@
 
 [testenv:plugin-sanity-check]
 # perform tempest plugin sanity
+basepython = python3
 whitelist_externals = bash
 commands =
   bash tools/tempest-plugin-sanity.sh