Make available to build docs with python3

On Ubuntu 16.04 a tox package is for python3 by default
and this causes errors in building docs in tox enviroment.

In these changes iterators are replaced with lists where it's needed.
Also external command calls result are decoded from bytes to unicode.

Change-Id: I88ef54405b4bc13c269bdda55ae8289676311ee1

api-guide/source/conf.py

@@ -88,7 +88,7 @@ gitsha = subprocess.Popen(
 
 # source tree
 pwd = subprocess.Popen(
-    "pwd", stdout=subprocess.PIPE).communicate()[0].strip('\n')
+    "pwd", stdout=subprocess.PIPE).communicate()[0].decode().strip('\n')
 
 # html_context allows us to pass arbitrary values into the html template
 html_context = {"pwd": pwd,