Replace deprecated library function os.popen() with subprocess

os.popen() is deprecated since python 2.6.
Resolved with use of subprocess module

Change-Id: I4b6469bf17a26389bcc6d844140a14f45896b9ff
Closes-Bug: #1529836

doc/source/conf.py

@@ -15,6 +15,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import subprocess
 import sys
 
 import os
@@ -145,8 +146,10 @@ html_static_path = ['_static']
 # If not '', a 'Last updated on:' timestamp is inserted at every page bottom,
 # using the given strftime format.
 #html_last_updated_fmt = '%b %d, %Y'
-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.