Fix test incompatibility with python-3.7+

platform.linux_distribution was removed in python-3.7.

Related: #2175198
(cherry picked from commit a517fa496c7a947b0d943fdf90a70b9d6eb312a8)

tests/functions_test/dmpd_library.py

@@ -155,11 +155,11 @@ def dist_release():
     """
     Find out the release number of Linux distribution.
     """
-    dist = platform.linux_distribution()
-    if not dist or dist[1] == "":
+    dist = platform.release()
+    if not dist:
         _print("WARN: dist_release() - Could not determine dist release")
         return None
-    return dist[1]
+    return dist
 
 
 def dist_ver():