doc: fix Sphinx conf.py for Python3

Fix Sphinx configuration file (conf.py) to make it compatible with
Python3. Popen opens channel in binary mode by default on Python3. Need
to specify text mode, with universal_newlines option which is also
available on Python2.

doc/source/conf.py

@@ -59,11 +59,11 @@ def get_version_release_from_git():
     Returns project version and release as string from 'git' repository data.
     """
     DEVNULL = open(os.devnull, 'w')
-    pipe = Popen('git describe --tags --abbrev=0', stdout=PIPE, stderr=DEVNULL, shell=True)
+    pipe = Popen('git describe --tags --abbrev=0', stdout=PIPE, stderr=DEVNULL, shell=True, universal_newlines=True)
     git_current_tag = pipe.stdout.read()
-    pipe = Popen('git describe --tags', stdout=PIPE, stderr=DEVNULL, shell=True)
+    pipe = Popen('git describe --tags', stdout=PIPE, stderr=DEVNULL, shell=True, universal_newlines=True)
     git_current_desc = pipe.stdout.read()
-    pipe = Popen('git rev-parse --abbrev-ref HEAD', stdout=PIPE, stderr=DEVNULL, shell=True)
+    pipe = Popen('git rev-parse --abbrev-ref HEAD', stdout=PIPE, stderr=DEVNULL, shell=True, universal_newlines=True)
     git_current_branch = pipe.stdout.read()
 
     if git_current_desc: