Use sqlalchemy text for ensuring session existence

Textual SQL expressions should be passed using text() method. Passing
them as a simple string is deprecated and can be no longer used with
modern sqlalchemy.

This also squashes patch [1] which replaces pysnmp[1] that is no longer
maintained with a fork pysnmp-lextudio[2] to resolve circular dependency

[1] https://review.opendev.org/c/openstack/vitrage/+/918415
[2] https://github.com/lextudio/pysnmp

Depends-On: https://review.opendev.org/c/openstack/vitrage-tempest-plugin/+/927167
Co-Authored-By: Takashi Kajinami <kajinamit@oss.nttdata.com>
Change-Id: Ied45b260415826a09c9c2358d37c34452ab10f32

requirements.txt

@@ -28,7 +28,7 @@ stevedore>=1.28.0 # Apache-2.0
 voluptuous>=0.11.1 # BSD License
 SQLAlchemy>=1.2.5 # MIT
 sympy>=1.1.1 # BSD
-pysnmp>=4.4.4 # BSD
+pysnmp-lextudio>=5.0.0  # BSD
 PyJWT>=1.6.0 # MIT
 osprofiler>=2.0.0 # Apache-2.0
 keystoneauth1>=3.6.2 # Apache-2.0

vitrage/storage/__init__.py

@@ -15,6 +15,7 @@
 from oslo_config import cfg
 from oslo_db.sqlalchemy import enginefacade
 from oslo_log import log
+from sqlalchemy import text
 from stevedore import driver
 import tenacity
 import threading
@@ -56,7 +57,7 @@ def get_connection_from_config():
         """Return an open connection to the database."""
         conn = mgr.driver(url)
         with enginefacade.reader.using(_CONTEXT) as session:
-            session.execute('SELECT 1;')
+            session.execute(text('SELECT 1;'))
         return conn
 
     return _get_connection()