Silence python3.6 cryptography deprecation warnings

```
/opt/**********/st2/lib/python3.6/site-packages/st2client/shell.py:27: CryptographyDeprecationWarning: Python 3.6 is no longer supported by the Python core team. Therefore, support for it is deprecated in cryptography. The next release of cryptography (40.0) will be the last to support Python 3.6.
  from cryptography.utils import CryptographyDeprecationWarning
```
in CLI and failing py3.6 tests

st2common/tests/integration/test_logging.py

@@ -15,6 +15,12 @@
 
 from __future__ import absolute_import
 
+# Ignore CryptographyDeprecationWarning warnings which appear on Python 3.6
+# TODO: Remove after dropping python3.6
+import warnings
+
+warnings.filterwarnings("ignore", message="Python 3.6 is no longer supported")
+
 import os
 import sys
 import signal