NSO Pyang Python3 support changes

pyang python3 support

# If you want to use Python 3 alonge with old NSO version which contains pyang issue to support python3 here are few steps
# 1. we are convering the python syntax to support Python2 and Python3
#    a. PEP 3113
#    b. importing StringIO
# 2. we are not planning to correct any ned's, we request to collect latest neds to support python3

File: NSO-<version>/lib/pyang/pyang/plugins/html.py
# Existing:
from cStringIO import StringIO
# Updates:
if sys.version_info.major == 3:
    from io import StringIO
else:
    from cStringIO import StringIO

# Existing:
def wrap_link(ctx,(stmt, module),stmt_arg,link_name=None):
# Updates:
def wrap_link(ctx,stmt_module,stmt_arg,link_name=None):
    stmt, module = stmt_module
    
File: NSO-<version>/lib/pyang/pyang/plugins/rest_doc.py
# Existing:
import StringIO
# Updates:
import io as StringIO

File: NSO-<version>/lib/pyang/pyang/plugins/tailf.py
# Existing:
def change_prefix(tokname, s)
# Updates:
def change_prefix(tokname_s)
    tokname, s = tokname_s