Server: Replace undocumented logging.warn() with logging.warning()

* The warn() variant was never documented and was deprecated in Py3.3.
* Correct variant warning() worked in all Python versions since logging
  module inclusion.

Details: https://bugs.python.org/issue13235



Signed-off-by: Pavel Kácha <ph@cesnet.cz>

warden3/warden_server/warden_server.py

@@ -605,7 +605,7 @@ class MySQL(ObjectBase):
             with attempt as db:
                 rows = db.query("".join(query), params).fetchall()
                 if len(rows) > 1:
-                    self.log.warn(
+                    self.log.warning(
                         "get_client_by_name: query returned more than one result (cert_names = %s, name = %s, secret = %s): %s" % (
                             cert_names, name, secret, ", ".join([str(Client(**row)) for row in rows])))
                     return None