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: I0cc16f50fef738594c123a27f34a517dd002e4f5

doc/source/conf.py

@@ -15,6 +15,7 @@
 from __future__ import unicode_literals
 
 import os
+import subprocess
 import sys
 
 import pbr.version
@@ -147,8 +148,10 @@ modindex_common_prefix = ['keystoneauth1.']
 
 # 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.