Replace deprecated library function os.popen() with subprocess

os.popen() is deprecated since version 2.6. Resolved with use of
subprocess module.
Closes-bug: #1529836

Change-Id: Ibdb8f13462e8416097006e19eb2214425364910e

doc/source/conf.py

@@ -12,6 +12,7 @@
 # serve to show the default.
 
 import sys
+import subprocess
 import os
 
 # If extensions (or modules to document with autodoc) are in another directory,
@@ -124,8 +125,10 @@ html_theme = 'nature'
 
 # If not '', a 'Last updated on:' timestamp is inserted at every page bottom,
 # using the given strftime format.
-git_cmd = "git log --pretty=format:'%ad, commit %h' --date=local -n1"
-html_last_updated_fmt = os.popen(git_cmd).read()
+git_cmd = ["git", "log", "--pretty=format:'%ad, commit %h'", "--date=local",
+           "-n1"]
+html_last_updated_fmt = subprocess.Popen(git_cmd,
+                                         stdout=subprocess.PIPE).communicate()[0]
 
 # If true, SmartyPants will be used to convert quotes and dashes to
 # typographically correct entities.