Remove hacking rules for python 2/3 compatibility

The Python 2.7 Support has been dropped since Ussuri.
So remove hacking rules for compatibility between python 2 and 3.

- [N325] str() and unicode() cannot be used on an exception.
         Remove or use six.text_type()
- [N327] Do not use xrange(). xrange() is not compatible with Python 3.
         Use range() or six.moves.range() instead.
- [N344] Python 3: do not use dict.iteritems.
- [N345] Python 3: do not use dict.iterkeys.
- [N346] Python 3: do not use dict.itervalues.

See also line 414 in https://etherpad.opendev.org/p/nova-victoria-ptg

Change-Id: If4335b2e8ef5bbabba37598110c1aa8269635c2f
Implements: blueprint six-removal
Signed-off-by: Takashi Natsume <takanattie@gmail.com>

tox.ini

@@ -288,7 +288,6 @@ extension =
   N323 = checks:check_explicit_underscore_import
   N324 = checks:use_jsonutils
   N332 = checks:check_api_version_decorator
-  N325 = checks:CheckForStrUnicodeExc
   N326 = checks:CheckForTransAdd
   N334 = checks:assert_true_or_false_with_in
   N336 = checks:dict_constructor_with_list_copy
@@ -300,10 +299,6 @@ extension =
   N350 = checks:check_policy_registration_in_central_place
   N351 = checks:check_policy_enforce
   N343 = checks:check_doubled_words
-  N344 = checks:check_python3_no_iteritems
-  N345 = checks:check_python3_no_iterkeys
-  N346 = checks:check_python3_no_itervalues
-  N327 = checks:check_python3_xrange
   N348 = checks:no_os_popen
   N352 = checks:no_log_warn
   N349 = checks:CheckForUncalledTestClosure